diff --git a/ngcp/dp.lua b/ngcp/dp.lua
index 731a4fa..deb0657 100644
--- a/ngcp/dp.lua
+++ b/ngcp/dp.lua
@@ -8,6 +8,15 @@ NGCPDomainPrefs = {
 }
 NGCPDomainPrefs_MT = { __index = NGCPDomainPrefs }
 
+NGCPDomainPrefs_MT.__tostring = function ()
+        local output = ''
+        local xavp = NGCPXAvp:new('caller','dom_prefs')
+        output = string.format("caller_dom_prefs:%s\n", tostring(xavp))
+        xavp = NGCPXAvp:new('callee','dom_prefs')
+        output = output .. string.format("callee_dom_prefs:%s\n", tostring(xavp))
+        return output
+    end
+
     function NGCPDomainPrefs:new(config)
         local t = {
             config = config,
diff --git a/ngcp/ngcp.lua b/ngcp/ngcp.lua
index 3098f2d..ab00266 100644
--- a/ngcp/ngcp.lua
+++ b/ngcp/ngcp.lua
@@ -53,6 +53,15 @@ NGCP = {
 }
 NGCP_MT = { __index = NGCP }
 
+NGCP_MT.__tostring = function (t)
+    local k,v
+    output = ''
+    for k,v in pairs(t.prefs) do
+        output = output .. tostring(v)
+    end
+    return output
+end
+
     function NGCP:new()
         local t = NGCP.init()
         setmetatable( t, NGCP_MT )
diff --git a/ngcp/pp.lua b/ngcp/pp.lua
index d90f82c..978e6f2 100644
--- a/ngcp/pp.lua
+++ b/ngcp/pp.lua
@@ -8,6 +8,15 @@ NGCPPeerPrefs = {
 }
 NGCPPeerPrefs_MT = { __index = NGCPPeerPrefs }
 
+NGCPPeerPrefs_MT.__tostring = function ()
+        local output = ''
+        local xavp = NGCPXAvp:new('caller','peer_prefs')
+        output = string.format("caller_peer_prefs:%s\n", tostring(xavp))
+        xavp = NGCPXAvp:new('callee','peer_prefs')
+        output = output .. string.format("callee_peer_prefs:%s\n", tostring(xavp))
+        return output
+    end
+
     function NGCPPeerPrefs:new(config)
         local t = {
             config = config,
diff --git a/ngcp/rp.lua b/ngcp/rp.lua
index 9cf7526..41beb3d 100644
--- a/ngcp/rp.lua
+++ b/ngcp/rp.lua
@@ -8,6 +8,15 @@ NGCPRealPrefs = {
  }
 NGCPRealPrefs_MT = { __index = NGCPRealPrefs }
 
+NGCPRealPrefs_MT.__tostring = function ()
+        local output = ''
+        local xavp = NGCPXAvp:new('caller','real_prefs')
+        output = string.format("caller_real_prefs:%s\n", tostring(xavp))
+        xavp = NGCPXAvp:new('callee','real_prefs')
+        output = output .. string.format("callee_real_prefs:%s\n", tostring(xavp))
+        return output
+    end
+
     function NGCPRealPrefs:new()
         local t = {}
         -- creates xavp real
diff --git a/ngcp/up.lua b/ngcp/up.lua
index 5f876cf..58a6013 100644
--- a/ngcp/up.lua
+++ b/ngcp/up.lua
@@ -8,6 +8,15 @@ NGCPUserPrefs = {
 }
 NGCPUserPrefs_MT = { __index = NGCPUserPrefs }
 
+NGCPUserPrefs_MT.__tostring = function ()
+        local output = ''
+        local xavp = NGCPXAvp:new('caller','usr_prefs')
+        output = string.format("caller_usr_prefs:%s\n", tostring(xavp))
+        xavp = NGCPXAvp:new('callee','usr_prefs')
+        output = output .. string.format("callee_usr_prefs:%s\n", tostring(xavp))
+        return output
+    end
+
     function NGCPUserPrefs:new(config)
         local t = {
             config = config,
diff --git a/ngcp/xavp.lua b/ngcp/xavp.lua
index a14a05d..bcf157e 100644
--- a/ngcp/xavp.lua
+++ b/ngcp/xavp.lua
@@ -30,6 +30,17 @@ NGCPXAvp_MT = {
                 error("value is not a number or string")
             end
         end
+        NGCPXAvp_MT.__tostring = function (t)
+            local l,k,v
+            local output
+
+            l = sr.xavp.get(t.name, 0)
+            if l then
+                output = table.tostring(l)
+            end
+            sr.log("dbg", string.format("output:%s", output))
+            return output
+        end
         setmetatable( t, NGCPXAvp_MT )
         return t
     end
@@ -103,17 +114,5 @@ NGCPXAvp_MT = {
         sr.pv.unset(string.format("$xavp(%s)", self.name))
         sr.pv.sets(string.format("$xavp(%s=>dummy)", self.name), self.level)
     end
-
-    function NGCPXAvp.__tostring(xavp)
-        local l,k,v
-        local output
-
-        l = sr.xavp.get(xavp.name, 0)
-        if l then
-            output = table.tostring(l)
-        end
-        sr.log("dbg", string.format("output:%s", output))
-        return output
-    end
 -- class
 --EOF
\ No newline at end of file
diff --git a/tests/ngcp.lua b/tests/ngcp.lua
index 797b709..28bbe08 100644
--- a/tests/ngcp.lua
+++ b/tests/ngcp.lua
@@ -504,6 +504,10 @@ TestNGCP = {} --class
         assertEquals(sr.pv.get("$avp(s:caller_ip_header)"), nil)
     end
 
+    function TestNGCP:test_tostring()
+        assertEquals(tostring(self.ngcp), 'caller_usr_prefs:{dummy="caller"}\ncallee_usr_prefs:{dummy="callee"}\ncaller_real_prefs:{dummy="caller"}\ncallee_real_prefs:{dummy="callee"}\ncaller_peer_prefs:{dummy="caller"}\ncallee_peer_prefs:{dummy="callee"}\ncaller_dom_prefs:{dummy="caller"}\ncallee_dom_prefs:{dummy="callee"}\n')
+    end
+
     function TestNGCP:test_log_var()
         self:test_caller_usr_load()
         self.ngcp:log_var()
diff --git a/tests/ngcp_dp.lua b/tests/ngcp_dp.lua
index 15505fb..4dfd702 100644
--- a/tests/ngcp_dp.lua
+++ b/tests/ngcp_dp.lua
@@ -156,5 +156,15 @@ TestNGCPDomainPrefs = {} --class
         assertEquals(sr.pv.get("$xavp(callee_dom_prefs=>foo)"),"foo")
         assertEquals(sr.pv.get("$xavp(callee_dom_prefs=>dummy)"),"callee")
     end
+
+    function TestNGCPDomainPrefs:test_tostring()
+        local callee_xavp = NGCPDomainPrefs:xavp('callee')
+        callee_xavp("testid",1)
+        callee_xavp("foo","foo")
+        local caller_xavp = NGCPDomainPrefs:xavp('caller')
+        caller_xavp("other",1)
+        caller_xavp("otherfoo","foo")
+        assertEquals(tostring(self.d), 'caller_dom_prefs:{other=1,otherfoo="foo",dummy="caller"}\ncallee_dom_prefs:{dummy="callee",testid=1,foo="foo"}\n')
+    end
 -- class TestNGCPDomainPrefs
 --EOF
\ No newline at end of file
diff --git a/tests/ngcp_pp.lua b/tests/ngcp_pp.lua
index eba7f22..5a820c7 100644
--- a/tests/ngcp_pp.lua
+++ b/tests/ngcp_pp.lua
@@ -157,5 +157,15 @@ TestNGCPPeerPrefs = {} --class
         assertEquals(sr.pv.get("$xavp(callee_peer_prefs=>foo)"),"foo")
         assertEquals(sr.pv.get("$xavp(callee_peer_prefs=>dummy)"),"callee")
     end
+
+    function TestNGCPPeerPrefs:test_tostring()
+        local callee_xavp = NGCPPeerPrefs:xavp('callee')
+        callee_xavp("testid",1)
+        callee_xavp("foo","foo")
+        local caller_xavp = NGCPPeerPrefs:xavp('caller')
+        caller_xavp("other",1)
+        caller_xavp("otherfoo","foo")
+        assertEquals(tostring(self.d), 'caller_peer_prefs:{other=1,otherfoo="foo",dummy="caller"}\ncallee_peer_prefs:{dummy="callee",testid=1,foo="foo"}\n')
+    end
 -- class TestNGCPPeerPrefs
 --EOF
\ No newline at end of file
diff --git a/tests/ngcp_rp.lua b/tests/ngcp_rp.lua
index d8fed69..f808d02 100644
--- a/tests/ngcp_rp.lua
+++ b/tests/ngcp_rp.lua
@@ -174,9 +174,9 @@ TestNGCPRealPrefs = {} --class
         callee_xavp("foo","foo")
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>foo)"),"foo")
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>dummy)"),"caller")
-        
+
         self.real:clean()
-        
+
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>dummy)"),"caller")
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>dummy)"),"callee")
     end
@@ -187,19 +187,19 @@ TestNGCPRealPrefs = {} --class
 
         callee_xavp("testid",1)
         callee_xavp("foo","foo")
-        
+
         caller_xavp("other",1)
         caller_xavp("otherfoo","foo")
-        
+
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>testid)"),1)
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>foo)"),"foo")
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>dummy)"),"caller")
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>other)"),1)
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>otherfoo)"),"foo")
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>dummy)"),"callee")
-        
+
         self.real:clean('callee')
-        
+
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>dummy)"),'caller')
         assertFalse(sr.pv.get("$xavp(callee_real_prefs=>testid)"))
         assertFalse(sr.pv.get("$xavp(callee_real_prefs=>foo)"))
@@ -214,19 +214,19 @@ TestNGCPRealPrefs = {} --class
 
         callee_xavp("testid",1)
         callee_xavp("foo","foo")
-        
+
         caller_xavp("other",1)
         caller_xavp("otherfoo","foo")
-        
+
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>testid)"),1)
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>foo)"),"foo")
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>dummy)"),"caller")
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>other)"),1)
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>otherfoo)"),"foo")
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>dummy)"),"callee")
-        
+
         self.real:clean('caller')
-        
+
         assertEquals(sr.pv.get("$xavp(caller_real_prefs=>dummy)"),"caller")
         assertFalse(sr.pv.get("$xavp(caller_real_prefs=>other)"))
         assertFalse(sr.pv.get("$xavp(caller_real_prefs=>otherfoo)"))
@@ -234,5 +234,15 @@ TestNGCPRealPrefs = {} --class
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>foo)"),"foo")
         assertEquals(sr.pv.get("$xavp(callee_real_prefs=>dummy)"),"callee")
     end
+
+    function TestNGCPRealPrefs:test_tostring()
+        local callee_xavp = NGCPRealPrefs:xavp("callee")
+        local caller_xavp = NGCPRealPrefs:xavp("caller")
+        callee_xavp("testid",1)
+        callee_xavp("foo","foo")
+        caller_xavp("other",1)
+        caller_xavp("otherfoo","foo")
+        assertEquals(tostring(self.real),'caller_real_prefs:{other=1,otherfoo="foo",dummy="caller"}\ncallee_real_prefs:{dummy="callee",testid=1,foo="foo"}\n')
+    end
 -- class TestNGCPRealPrefs
 --EOF
\ No newline at end of file
diff --git a/tests/ngcp_up.lua b/tests/ngcp_up.lua
index a517495..8d7eac8 100644
--- a/tests/ngcp_up.lua
+++ b/tests/ngcp_up.lua
@@ -107,7 +107,7 @@ TestNGCPUserPrefs = {} --class
         assertEquals(sr.pv.get("$xavp(callee_usr_prefs=>ac)"),"1")
         assertItemsEquals(keys, {"account_id", "cli", "cc", "ac"})
     end
-    
+
     function TestNGCPUserPrefs:test_clean()
         local xavp = NGCPUserPrefs:xavp('callee')
         xavp("testid",1)
@@ -163,5 +163,15 @@ TestNGCPUserPrefs = {} --class
         assertEquals(sr.pv.get("$xavp(callee_usr_prefs=>foo)"),"foo")
         assertEquals(sr.pv.get("$xavp(callee_usr_prefs=>dummy)"),"callee")
     end
+
+    function TestNGCPUserPrefs:test_tostring()
+        local callee_xavp = NGCPUserPrefs:xavp('callee')
+        callee_xavp("testid",1)
+        callee_xavp("foo","foo")
+        local caller_xavp = NGCPUserPrefs:xavp('caller')
+        caller_xavp("other",1)
+        caller_xavp("otherfoo","foo")
+        assertEquals(tostring(self.d),'caller_usr_prefs:{other=1,otherfoo="foo",dummy="caller"}\ncallee_usr_prefs:{dummy="callee",testid=1,foo="foo"}\n')
+    end
 -- class TestNGCPUserPrefs
 --EOF
\ No newline at end of file
diff --git a/tests/ngcp_xavp.lua b/tests/ngcp_xavp.lua
index 25d830f..071f324 100644
--- a/tests/ngcp_xavp.lua
+++ b/tests/ngcp_xavp.lua
@@ -103,7 +103,7 @@ TestNGCPXAvp = {} --class
 
     function TestNGCPXAvp:test_tostring()
         local xavp = NGCPXAvp:new("caller", "peer", {})
-        assertEquals(xavp:__tostring(), '{dummy="caller"}')
+        assertEquals(tostring(xavp), '{dummy="caller"}')
     end
 
     function TestNGCPXAvp:test_keys()