Calling 9904, nothing happens because the SIP user "arno" doesn't exist in sip.conf:

    Set remote console verbosity to 9
      == Using SIP RTP CoS mark 5
        -- Executing [9904@tixe:1] Dial("SIP/amc-00000028", "SIP/932&SIP/amc&SIP/arno&SIP/930,30") in new stack
      == Using SIP RTP CoS mark 5
      == Using SIP RTP CoS mark 5
      == Using SIP RTP CoS mark 5
     
     
< Nothing happens, nothing rings here>
     
# ADDING user [arno] to sip.conf (dummy/fake)

     pbx*CLI> sip reload
          
# Calling 9904, it now calls. SIP user "arno" does exist in sip.conf, but isn't registered.
     
     Reloading SIP
      == Parsing '/etc/asterisk/sip.conf': Found
      == Using SIP CoS mark 4
      == Using SIP RTP CoS mark 5
        -- Executing [9904@tixe:1] Dial("SIP/amc-0000002b", "SIP/932&SIP/amc&SIP/arno&SIP/930,30") in new stack
      == Using SIP RTP CoS mark 5
      == Using SIP RTP CoS mark 5
    [Nov 20 13:28:52] WARNING[4875][C-00000012]: app_dial.c:2437 dial_exec_full: Unable to create channel of type 'SIP' (cause 20 - Subscriber absent)
      == Using SIP RTP CoS mark 5
        -- Called SIP/932
        -- Called SIP/amc
        -- Called SIP/930
        -- Got SIP response 486 "Busy Here" back from 192.168.1.107:5060
        -- SIP/amc-0000002d is busy
        -- SIP/930-0000002e is ringing
        -- SIP/932-0000002c is ringing
        -- SIP/932-0000002c is ringing
      == Spawn extension (tixe, 9904, 1) exited non-zero on 'SIP/amc-0000002b'

# OK.


# Back to first case with nonexistant SIP peer "arno": When calling 9904 from an IAX2 trunk, the trunk stays busy forever:
# SETUP:   [remotepbx] <-> IAX2 trunk <-> [localpbx] -> extension 9904
# Log below is from localpbx, verbosity 0

    [Nov 20 12:43:35] WARNING[3545]: channel.c:1309 __ast_queue_frame: Exceptionally long voice queue length queuing to IAX2/ahlecuador-4809
    [Nov 20 12:43:36] WARNING[3548]: channel.c:1309 __ast_queue_frame: Exceptionally long voice queue length queuing to IAX2/ahlecuador-4809
    [Nov 20 12:43:37] WARNING[3544]: channel.c:1309 __ast_queue_frame: Exceptionally long voice queue length queuing to IAX2/ahlecuador-4809
    [Nov 20 12:43:38] WARNING[3549]: channel.c:1309 __ast_queue_frame: Exceptionally long voice queue length queuing to IAX2/ahlecuador-4809
     
    [Nov 20 12:33:26] WARNING[1222]: chan_iax2.c:3552 __attempt_transmit: Max retries exceeded to host 85.164.151.220 on IAX2/ahlecuador-5666 (type = 6, subclass = 11, ts=1769955, seqno=10)
    [Nov 20 12:33:26] WARNING[1223]: chan_iax2.c:3552 __attempt_transmit: Max retries exceeded to host 85.164.151.220 on IAX2/ahlecuador-13906 (type = 6, subclass = 2, ts=1700976, seqno=0)
    [Nov 20 12:33:33] WARNING[1224]: chan_iax2.c:3552 __attempt_transmit: Max retries exceeded to host 85.164.151.220 on IAX2/ahlecuador-531 (type = 6, subclass = 11, ts=139995, seqno=22)
    [Nov 20 12:33:35] WARNING[1217]: chan_iax2.c:3552 __attempt_transmit: Max retries exceeded to host 85.164.151.220 on IAX2/ahlecuador-13906 (type = 6, subclass = 11, ts=1709953, seqno=1)
    [Nov 20 12:33:36] WARNING[1223]: chan_iax2.c:3552 __attempt_transmit: Max retries exceeded to host 85.164.151.220 on IAX2/ahlecuador-5666 (type = 6, subclass = 11, ts=1779954, seqno=11)
    [Nov 20 12:33:40] WARNING[1218]: chan_iax2.c:3552 __attempt_transmit: Max retries exceeded to host 85.164.151.220 on IAX2/ahlecuador-531 (type = 6, subclass = 2, ts=146997, seqno=23)
     
# and so on.
# Actually have to restart localpbx asterisk service to get the trunk going again