hi to all
we have a voip system in our local network and it work properly internally. external user connect by l2tp to us. every things is ok and they can ping every where they must like sip server ip. soft-phone register to sip server properly too. but after successful call, voice is one way( internal caller voice not send to external user)
apologize for my bad English writing. let me know if any thing is unclear.

Check your firewall, and when using NAT check your device settings.
This is not related to l2tp but it is a very common problem with SIP on any router, especially with NAT.

Have you tried that?

Check your firewall, and when using NAT check your device settings.

What do you mean