If you are able to send data, it means that path from your application to their application is clear and when they send back MDN, it will be using same session and hence you recieve it successfully.
If customer is getting timeout while sending data, it could be:
Their firewall might not be open and hence data is not going out of their network. - Request them to run traceroute command to confirm the same.
Firewall on your side might not be open - Check with your firewall team for the same
Transaction hitting network, but not reaching internal servers. - Increase log level in RI to confirm if transactions are hitting.
If you have already performed these checks, pl let me know. I can suggest further checks.