How to change the IPs coming from VPN to same IPs of LAN

Hello,
I need your support for the for the following problem.
Port 1 = Connect to internet
Port 2 = 172.16.1.1
IP PBX = 172.16.1.250
VPN Connection Pool (10.10.40.2 - 10.10.40.10)
The problem is, once connecting VPN on my mobile and running calling app, The PBX recognize my mobile IP ADDRESS as 172.16.1.1, which is the IP ADDRESS of Port 2 of mikrotik.
my request is how to force Mikrotik router to add different IP from my configuration ex. 172.16.1.50.
Thanks for your support
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NAT rule which changes SRC address for VPN sourced connection going to PBS to the IP you wish.