How to check whether pots are open or not, and whether UDP traffic is allowed or not - between mkrotik routes over the VPN channel?

Question

VPN

Hello.
Please tell me how to check if the ports are open or not, and whether UDP traffic is allowed or not - between mkrotik routes over the VPN channel?
And that is, two PBXs and they communicate with each other by ip and port, one PBX No. 1 was able to connect to PBX No. 2 by ip address, and PBX No. 2 cannot connect to PBX No. 1.
PBXs are located on different subnets and are connected via a VPN channel raised on mikrotik routers.
PBXs are pinged from opposite subnets
. VPN 1C also works there, and there are no problems either.

Answer 1

Such things are checked by telnet indicating the ip and port. Command like one “telnet ip port”

Answer 2

Checked through a telnet, ports are closed, but one automatic telephone exchange is connected to another. It's strange like that. Thanks, I'll keep thinking.