How is the remote office setup?
It is via VPN/Private ip or is it public?
What settings do you have in the system for the ip phones (private/public/public w firewall)?

I've had a similar issue with 4.14 software on os7100 where 1 phone would work and the others would have no speech.
the working phone shifted based on which phone was used first, and then once the nat translation in the router expired then the next handset would work.
This was fixed in 4.14k.

You may need to wireshark trace when the call fails.
The issue is having such old software versions and hardware you're not going to get any help from samsung, even if you upgrade due to the mcp being EOL