prevent Source Ip to change

hi everyone

please help

I have 2 offices connected over PPTP VPN

Office X 192.168.8.0/22 Mikrotik
Office Y 192.168.15.0/24 Draytik

example
I have IP telephony at office Y telephone IP 192.168.15.66 at office X I Have the PBX once the telephone is registered it shows the IP Address of Mikrotik (office X gateway IP)
if I switch the scenario the original IP Address of the telephone will be registered

can I prevent this from changing