Help - Route 2 Segment under 2 Gateway.

RouterOS Beginner Basics
1

Hello,
Before I start, sorry for my bad english.

Need suggestion or solution for my problem.
I have PABX Devices and module with IP 192.168.10.0/24, these PABX devices connected through 192.168.10.1 gateway (Cisco).
Also I have SIP Trunk device from the ISP with IP 10.18.31.182/30 these SIP Trunk connected through 10.18.31.181 (SBC).

The Problem is, need to connect these 2 segments IP with Mikrotik (can interact each other) without touching or configuring the Cisco device (192.168.10.1/24) or the SBC (10.18.31.181/30) .
What i’ve done, connected :
The eth2 with 10.18.31.182/30.
The eth3 with 192.168.10.200/24.

Need help and suggestion for the routing and the firewall thing.

Thank you.

2

Problem is, devices use their default gateway to access other subnets. And just because you add router connected to both subnets, it won’t magically start routing between them. The router itself would be for it, after all, routing is its life. But other devices won’t start to use it as gateway when they already have another.