Only when calling from estensione: unable to create channel of type 'SIP' (cause 20 - Subscriber absent)

Hello, I own from many years a freepbx server well configured. Now, I added an external extension and forwarded the requested ports . All seems ok but from time to time both LAN extensions and the outside extension , and only when I generate a call saying the service is not available at the moment . I checked the log and here it is : what can be the reason? Also older extensions are affected by this issue.

[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Goto (macro-outbound-callerid,s,14)
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:14] ExecIf(“SIP/57-00000050”, “1?Set(CALLERID(all)=003901998861)”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:15] ExecIf(“SIP/57-00000050”, “0?Set(CALLERID(all)=)”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:16] ExecIf(“SIP/57-00000050”, “0?Set(CALLERID(all)=)”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:17] ExecIf(“SIP/57-00000050”, “0?Set(CALLERPRES()=prohib_passed_screen)”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:18] Set(“SIP/57-00000050”, “CDR(outbound_cnum)=003901998861”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:19] Set(“SIP/57-00000050”, “CDR(outbound_cnam)=”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:12] GosubIf(“SIP/57-00000050”, “0?sub-flp-2,s,1()”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:13] Set(“SIP/57-00000050”, “OUTNUM=3936212873”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:14] Set(“SIP/57-00000050”, “custom=SIP/FreeVoipDeal”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:15] ExecIf(“SIP/57-00000050”, “1?Set(DIAL_TRUNK_OPTIONS=M(setmusic^musica)wW)”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:16] ExecIf(“SIP/57-00000050”, “0?Set(DIAL_TRUNK_OPTIONS=M(setmusic^musica)wWM(confirm))”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:17] Macro(“SIP/57-00000050”, “dialout-trunk-predial-hook,”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:1] MacroExit(“SIP/57-00000050”, “”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:18] GotoIf(“SIP/57-00000050”, “0?bypass,1”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:19] ExecIf(“SIP/57-00000050”, “1?Set(CONNECTEDLINE(num,i)=3936212873)”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:20] ExecIf(“SIP/57-00000050”, “1?Set(CONNECTEDLINE(name,i)=CID:003901998861)”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:21] GotoIf(“SIP/57-00000050”, “0?customtrunk”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:22] Dial(“SIP/57-00000050”, “SIP/FreeVoipDeal/3936212873,300,M(setmusic^musica)wW”) in new stack
[2016-04-20 16:31:12] WARNING[5103][C-00000038] app_dial.c: Unable to create channel of type ‘SIP’ (cause 20 - Subscriber absent)
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] app_dial.c: == Everyone is busy/congested at this time (1:0/0/1)
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:23] NoOp(“SIP/57-00000050”, “Dial failed for some reason with DIALSTATUS = CHANUNAVAIL and HANGUPCAUSE = 20”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:24] GotoIf(“SIP/57-00000050”, “0?continue,1:s-CHANUNAVAIL,1”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Goto (macro-dialout-trunk,s-CHANUNAVAIL,1)
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:1] Set(“SIP/57-00000050”, “RC=20”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:2] Goto(“SIP/57-00000050”, “20,1”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Goto (macro-dialout-trunk,20,1)
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:1] Goto(“SIP/57-00000050”, “continue,1”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Goto (macro-dialout-trunk,continue,1)
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:1] NoOp(“SIP/57-00000050”, “TRUNK Dial failed due to CHANUNAVAIL HANGUPCAUSE: 20 - failing through to other trunks”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:2] Set(“SIP/57-00000050”, “CALLERID(number)=57”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:6] Macro(“SIP/57-00000050”, “outisbusy,”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:1] Progress(“SIP/57-00000050”, “”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] pbx.c: – Executing [[email protected]:2] Playback(“SIP/57-00000050”, “all-circuits-busy-now,noanswer”) in new stack
[2016-04-20 16:31:12] VERBOSE[5103][C-00000038] file.c: – <SIP/57-00000050> Playing ‘all-circuits-busy-now.gsm’ (language ‘it’)