Starting with version 6.0rc14 we found a bug regarding functioning SIP VoIP PBXs.
The problem occurs both with 3CX Phone System that releases with Asterisk.
It seems that the changes introduced at the level of NAT somehow cause the fall of the SIP call after a few minutes.
Note that all systems always operated with static port mapping service and SIP ALG disabled and have always worked properly from version 5.0 routeros.
The problem was not present in the version 6.0rc13 and even in version 5.24.
In this situation it is impossible to properly operate an IP PBX behind routeros.
Please verify and fix the problem please.