chan_sip.c: SIP MESSAGE JUST IGNORED: ACK, BAD! BAD! BAD!

All the sudden, this error occurs :

Apr  9 13:34:48 ERROR[5749] chan_sip.c: We could NOT get the channel lock for SIP/505-08244318 - Call ID 3ec37f5e308574245e79b5240ed6dca8@10.0.0.155! 
Apr  9 13:34:48 ERROR[5749] chan_sip.c: SIP MESSAGE JUST IGNORED: ACK 
Apr  9 13:34:48 ERROR[5749] chan_sip.c: BAD! BAD! BAD!
Apr  9 13:34:49 ERROR[5749] chan_sip.c: We could NOT get the channel lock for SIP/505-08244318 - Call ID 3ec37f5e308574245e79b5240ed6dca8@10.0.0.155! 
Apr  9 13:34:49 ERROR[5749] chan_sip.c: SIP MESSAGE JUST IGNORED: ACK 
Apr  9 13:34:49 ERROR[5749] chan_sip.c: BAD! BAD! BAD!
Apr  9 13:34:50 ERROR[5749] chan_sip.c: We could NOT get the channel lock for SIP/505-08244318 - Call ID 3ec37f5e308574245e79b5240ed6dca8@10.0.0.155! 
Apr  9 13:34:50 ERROR[5749] chan_sip.c: SIP MESSAGE JUST IGNORED: ACK 
Apr  9 13:34:50 ERROR[5749] chan_sip.c: BAD! BAD! BAD!
Apr  9 13:35:41 ERROR[5749] chan_sip.c: We could NOT get the channel lock for SIP/505-08259b38 - Call ID 7b82cb33008cea3c3b3c8ca238513b85@10.0.0.155! 
Apr  9 13:35:41 ERROR[5749] chan_sip.c: SIP MESSAGE JUST IGNORED: ACK 
Apr  9 13:35:41 ERROR[5749] chan_sip.c: BAD! BAD! BAD!
Apr  9 13:35:42 ERROR[5749] chan_sip.c: We could NOT get the channel lock for SIP/505-08259b38 - Call ID 7b82cb33008cea3c3b3c8ca238513b85@10.0.0.155! 
Apr  9 13:35:42 ERROR[5749] chan_sip.c: SIP MESSAGE JUST IGNORED: ACK 
Apr  9 13:35:42 ERROR[5749] chan_sip.c: BAD! BAD! BAD!
Apr  9 13:35:43 ERROR[5749] chan_sip.c: We could NOT get the channel lock for SIP/505-08259b38 - Call ID 7b82cb33008cea3c3b3c8ca238513b85@10.0.0.155! 
Apr  9 13:35:43 ERROR[5749] chan_sip.c: SIP MESSAGE JUST IGNORED: ACK 
Apr  9 13:35:43 ERROR[5749] chan_sip.c: BAD! BAD! BAD!
Apr  9 13:38:20 ERROR[5749] chan_sip.c: We could NOT get the channel lock for SIP/507-08276f60 - Call ID 347f60630ab35b907d14e653788c0db2@10.0.0.157! 
Apr  9 13:38:20 ERROR[5749] chan_sip.c: SIP MESSAGE JUST IGNORED: BYE 
Apr  9 13:38:20 ERROR[5749] chan_sip.c: BAD! BAD! BAD!
Apr  9 13:38:21 ERROR[5749] chan_sip.c: We could NOT get the channel lock for SIP/507-08276f60 - Call ID 347f60630ab35b907d14e653788c0db2@10.0.0.157! 
Apr  9 13:38:21 ERROR[5749] chan_sip.c: SIP MESSAGE JUST IGNORED: BYE 
Apr  9 13:38:21 ERROR[5749] chan_sip.c: BAD! BAD! BAD!

and the phone becomes dead.
Then SIP is try to register the phone and then the phone becomes usable.

Asterisk v1.2.17
Phones:Planet 153T