Hello,
I've some years (+11 years) of experience on the Linux administration field, I'm new on Freelancer and maybe my profile isn't very complete...
But returning to your issue, some times the voice in one way (instead of two) is caused by firewall or routing problems.
Asterisk uses SIP with UDP (port 5060) transport for the session initiation and so on for the voice transport it uses RTP fom port 10000 to 20000 (in most cases).
When a problem as yours occurs, the first reason can be because the above described port configuration isn't comunicating fine.
Other possibility is the use of incorrect codec or voip client configuration in one of the ends.
Analyzing the environment is possible find the gap and solve it.
Have a nice day.
Thank you, regards.