This is when it works ok and I can hear 'this nubmer is invalid':

Mar 28 14:31:12 DEBUG[30766]: chan_h323.c:1181 external_rtp_create: Sending RTP 'US' 10.110.5.20:16468
Mar 28 14:31:12 DEBUG[30766]: chan_h323.c:1181 external_rtp_create: Sending RTP 'US' 10.110.5.20:16468
Mar 28 14:31:12 DEBUG[30766]: pbx.c:1774 ast_device_state_changed: Changing state for H323/ip$10.110.5.17:14393/10625
Mar 28 14:31:12 DEBUG[30768]: app_queue.c:380 changethread: Device 'H323/ip$10.110.5.17:14393/10625' changed to state '2'
Mar 28 14:31:12 DEBUG[30768]: app_queue.c:408 changethread: Device 'H323/ip$10.110.5.17:14393/10625' changed to state '2'
  == Starting H323/ip$10.110.5.17:14393/10625 at local,8,1 failed so falling back to exten 's'
  == Starting H323/ip$10.110.5.17:14393/10625 at local,s,1 still failed so falling back to context 'default'
    -- Sent into invalid extension 's' in context 'default' on H323/ip$10.110.5.17:14393/10625
Mar 28 14:31:12 DEBUG[30769]: pbx.c:1567 pbx_extension_helper: Launching 'Playback'
Mar 28 14:31:12 DEBUG[30766]: channel.c:1859 ast_set_read_format: Set channel H323/ip$10.110.5.17:14393/10625 to read format ulaw
Mar 28 14:31:12 DEBUG[30766]: channel.c:1826 ast_set_write_format: Set channel H323/ip$10.110.5.17:14393/10625 to write format ulaw
Mar 28 14:31:12 DEBUG[30766]: channel.c:1859 ast_set_read_format: Set channel H323/ip$10.110.5.17:14393/10625 to read format ulaw
Mar 28 14:31:12 DEBUG[30766]: channel.c:1826 ast_set_write_format: Set channel H323/ip$10.110.5.17:14393/10625 to write format ulaw
    -- Executing Playback("H323/ip$10.110.5.17:14393/10625", "invalid") in new stack
Mar 28 14:31:12 DEBUG[30769]: channel.c:1826 ast_set_write_format: Set channel H323/ip$10.110.5.17:14393/10625 to write format gsm
Mar 28 14:31:12 DEBUG[30769]: rtp.c:1323 ast_rtp_write: Ooh, format changed from unknown to ulaw
    -- Playing 'invalid' (language 'en')
Mar 28 14:31:12 DEBUG[30769]: channel.c:1461 ast_read: Dropping duplicate answer!
Mar 28 14:31:12 NOTICE[30769]: rtp.c:316 process_rfc3389: RFC3389 support incomplete.  Turn off on client if possible


This is when (on another attempt) I hear silence and experience dealdocks right afterwards:

Mar 28 14:34:57 DEBUG[30890]: chan_h323.c:1181 external_rtp_create: Sending RTP 'US' 10.110.5.20:19422
Mar 28 14:34:57 DEBUG[30890]: chan_h323.c:1181 external_rtp_create: Sending RTP 'US' 10.110.5.20:19422
Mar 28 14:34:57 DEBUG[30890]: pbx.c:1774 ast_device_state_changed: Changing state for H323/ip$10.110.5.17:15524/10639
Mar 28 14:34:57 DEBUG[30892]: app_queue.c:380 changethread: Device 'H323/ip$10.110.5.17:15524/10639' changed to state '2'
Mar 28 14:34:57 DEBUG[30892]: app_queue.c:408 changethread: Device 'H323/ip$10.110.5.17:15524/10639' changed to state '2'
  == Starting H323/ip$10.110.5.17:15524/10639 at local,8,1 failed so falling back to exten 's'
  == Starting H323/ip$10.110.5.17:15524/10639 at local,s,1 still failed so falling back to context 'default'
    -- Sent into invalid extension 's' in context 'default' on H323/ip$10.110.5.17:15524/10639
Mar 28 14:34:57 DEBUG[30893]: pbx.c:1567 pbx_extension_helper: Launching 'Playback'
    -- Executing Playback("H323/ip$10.110.5.17:15524/10639", "invalid") in new stack
	
This is the same but with commented out exten => i,1,Playback(invalid)
(deadlock still follows)
	
Mar 28 14:44:56 DEBUG[31250]: chan_h323.c:1181 external_rtp_create: Sending RTP 'US' 10.110.5.20:18720
Mar 28 14:44:56 DEBUG[31250]: chan_h323.c:1181 external_rtp_create: Sending RTP 'US' 10.110.5.20:18720
  == Starting H323/ip$10.110.5.17:3111/10654 at local,8,1 failed so falling back to exten 's'
  == Starting H323/ip$10.110.5.17:3111/10654 at local,s,1 still failed so falling back to context 'default'
Mar 28 14:44:56 WARNING[31253]: pbx.c:2299 ast_pbx_run: Channel 'H323/ip$10.110.5.17:3111/10654' sent into invalid extension 's' in context 'default', but no invalid handler