yeah VOIP is a funny beast during failover scenarios. I have experienced it first hand.
A past thread on this very topic that is the best.
http://forum.mikrotik.com/t/sip-client-cannot-re-register-in-the-sip-server-after-switching-isp-different-nat/115091/1
good thread on voip in general
http://forum.mikrotik.com/t/using-routeros-to-qos-your-network-2020-edition/66683/1
Other interesting MUM presentation on VOIP
https://mum.mikrotik.com/presentations/US17/presentation_4321_1496084451.pdf
https://www.youtube.com/watch?v=tM7wyKdnIKA&feature=youtu.be
Interesting url info for a script to let you know a link is down.
http://gregsowell.com/?p=819