I was having a very similar issue with deadlocks using chan_sip and realtime voicemail.
The patch to the 13 branch seemed to fix my problem.
https://gerrit.asterisk.org/#/c/3962/4
Also, I found, before the patch, that I was losing the Mailbox on the sip peer and would need to:
sip prune realtime peer 12345
sip show peer 12345 load
I would need to do that anywhere from 1 to 3 times, then the Mailbox would reload. As soon as I tried to access it, the mailbox would disappear from the peer again.
I ran the 13 branch with the patch applied since Friday. So far, nothing queueing in the taskprocessor.