I have a few questions, do you know what extension numbers are having issues with MWI?
Are they the same extension numbers each time?
The Cisco bug tool highlights several issues regarding SIP and MWI, but the one that stands out is CSCsd56683 - MWI will not be sent if phone is Deregistered.
From your 'sh tech' some phones are unregistering, are these the same phones that are having MWI problems?
The following 7961 phone has unregistered.
Feb 26 07:10:16.949 EST: %IPPHONE-6-UNREGISTER_ABNORMAL: ephone-71:SEP0017E08C6A7D
From your config, it would seem that this same 7961 phone is configured with voicemail - using SIP MWI.
ephone-dn 71 dual-line
number 671
label 671
description 99304671
name Workstation 21
call-forward busy 900
call-forward noan 900 timeout 10
corlist incoming STDCalls
mwi sip
!
Why have you elected to use SIP for MWI why not use SCCP?
