Dropped call

Hey folks,

Having very intermittent dropped call problems (less than 1 call a week on average) with a 100% sip install for a customer.
Here is the log of one of these calls as it hangs up (about 20 minutes into the call):

[Aug 28 15:15:16] VERBOSE[17938] logger.c:   == Spawn extension (macro-dialout-trunk, s, 20) exited non-zero on 'SIP/223-b7b01e18' in macro 'dialout-trunk'
[Aug 28 15:15:16] VERBOSE[17938] logger.c:   == Spawn extension (macro-dialout-trunk, s, 20) exited non-zero on 'SIP/223-b7b01e18'
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Executing [h@macro-dialout-trunk:1] Macro("SIP/223-b7b01e18", "hangupcall|") in new stack
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Executing [s@macro-hangupcall:1] ResetCDR("SIP/223-b7b01e18", "w") in new stack
[Aug 28 15:15:16] DEBUG[17938] app_macro.c: Executed application: ResetCDR
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Executing [s@macro-hangupcall:2] NoCDR("SIP/223-b7b01e18", "") in new stack
[Aug 28 15:15:16] DEBUG[17938] app_macro.c: Executed application: NoCDR
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Executing [s@macro-hangupcall:3] GotoIf("SIP/223-b7b01e18", "1?skiprg") in new stack
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Goto (macro-hangupcall,s,6)
[Aug 28 15:15:16] DEBUG[17938] app_macro.c: Executed application: GotoIf
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Executing [s@macro-hangupcall:6] GotoIf("SIP/223-b7b01e18", "1?skipblkvm") in new stack
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Goto (macro-hangupcall,s,9)
[Aug 28 15:15:16] DEBUG[17938] app_macro.c: Executed application: GotoIf
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Executing [s@macro-hangupcall:9] GotoIf("SIP/223-b7b01e18", "1?theend") in new stack
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Goto (macro-hangupcall,s,11)
[Aug 28 15:15:16] DEBUG[17938] app_macro.c: Executed application: GotoIf
[Aug 28 15:15:16] VERBOSE[17938] logger.c:     -- Executing [s@macro-hangupcall:11] Hangup("SIP/223-b7b01e18", "") in new stack
[Aug 28 15:15:16] VERBOSE[17938] logger.c:   == Spawn extension (macro-hangupcall, s, 11) exited non-zero on 'SIP/223-b7b01e18' in macro 'hangupcall'
[Aug 28 15:15:16] VERBOSE[17938] logger.c:   == Spawn extension (macro-hangupcall, s, 11) exited non-zero on 'SIP/223-b7b01e18'
[Aug 28 15:15:16] WARNING[2585] chan_sip.c: Remote host can't match request BYE to call '3e1270d00452c7c9072fd1eb65c5b7ea@XX.XX.XXX.XXX'. Giving up.

QoS is setup and working great otherwise, the ADSL link is good and stable, and cabling/network equipment is flawless. This happens to different peers as well, but not very often.
I'm at a loss to pinpoint the problem, any ideas?