with allow=g729 commented out.
------------------------------
-- Executing Set("SIP/PLCM300-01-fb9d", "SIP_CODEC=alaw") in new stack
    -- Executing Dial("SIP/PLCM300-01-fb9d", "Zap/g1/021230942:2|30") in new stack
    -- Requested transfer capability: 0x00 - SPEECH
    -- Called g1/021230942:2
May 31 13:57:13 NOTICE[8416]: chan_sip.c:2503 try_suggested_sip_codec: Changing codec to 'alaw' for this call because of ${SIP_CODEC) variable
    -- Channel 0/1, span 1 got hangup, cause 28
    -- Hungup 'Zap/1-1'
  == Everyone is busy/congested at this time (1:0/0/1)
    -- Executing Hangup("SIP/PLCM300-01-fb9d", "") in new stack

allow=g729 enabled
------------------
 Executing Set("SIP/PLCM300-01-41bb", "SIP_CODEC=alaw") in new stack
    -- Executing Dial("SIP/PLCM300-01-41bb", "Zap/g1/021230942:2|30") in new stack
May 31 13:56:00 WARNING[8410]: channel.c:2541 ast_request: No translator path exists for channel type Zap (native 76) to 256
May 31 13:56:00 NOTICE[8410]: app_dial.c:1055 dial_exec_full: Unable to create channel of type 'Zap' (cause 0 - Unknown)
  == Everyone is busy/congested at this time (1:0/0/1)
    -- Executing Hangup("SIP/PLCM300-01-41bb", "") in new stack