extensions.conf

...
exten => 1000,1,Dial(Zap/g0/6100)
exten => 1000,2,Hangup()
...


sip.conf
...
[1002]
type=friend
username=1002
secret=******
context=tronco-vono
dtmfmode=inband
host=dynamic
nat=1
canreinvite=no
disallow=all
allow=ulaw
allow=alaw
...


verbosity
...
Verbosity is at least 3
    -- Remote UNIX connection
    -- Executing Dial("SIP/1002-081f3e48", "Zap/g0/6100") in new stack
    -- Requested transfer capability: 0x00 - SPEECH
    -- Called g0/6100
    -- Zap/23-1 is proceeding passing it to SIP/1002-081f3e48
    -- Channel 0/23, span 1 got hangup request, cause 27
    -- Zap/23-1 is circuit-busy
    -- Hungup 'Zap/23-1'
  == Everyone is busy/congested at this time (1:0/1/0)
    -- Executing Hangup("SIP/1002-081f3e48", "") in new stack
  == Spawn extension (tronco-vono, 1000, 2) exited non-zero on 'SIP/1002-081f3e48'
...