Our Grandstream PBX tends to work pretty well most of the time, but once in a while, it gets into a state where it is no longer registering. It says it failed to register, our provider says they are not getting packets (and this is likely to be true, as we have two and we lose both).
Rebooting the Mikrotik fixes it every time. The PBX ceases to be able to register with the server, so I assume for whatever reason UDP packets are not getting through.
Try disabling the SIP protocol helper in the IP Firewall configuration.
We used to have a lot of Grandstream analog FXS gateways behind a lot of Mikrotik routers, and they would get exactly this type of behavior if the SIP protocol helper was left active in the Mikrotik.
With the last two updates my Grandstream system and phones has been having issues too. I just turned off my SIP ALG/Helper and there was no change either. I found out that if I do not connect the phones directly to the MikroTik router like another switch they register better, but calls fail through NAT occasionally still.
Make sure you allow 5060-5080 both up/rtcp from SIP provider to the pancakes and the bit that fixed registrations on my Grandstream UCM was also allowing 10001-20000 up from all addresses to my pabx. I had originally restricted it to my sip providers but coup.ms in particular did not seem to like that idea
