Atcom phone doesn't work on mikrotik network but siemens does? Can anyone explain this?

What do you mean it "doesn't work"? Could you give us some details.

The sip account times out under firwall connections

Make sure your Atcom phone is configured to use SIP NAT Keepalives.

Any SIP phone behind nat needs this to keep the "connections" or "xlat" or "nat xlat" or other definition of UDP transformations from expiring. Lots of firewalls need this.

I had Cisco 7960's in SIP mode unable to receive inbound calls periodically because I couldnt figure out how to set the keepalive and only the SIP registration interval kept establishing the UDP connections.

the phone doesn't have this option