No Audio over public IP

Hi, Everyone!

I had a question I was hoping you all could answer.
I’ve had a PBX online for about a year now and I have to make and receive calls over the public IP. Now, on some internet connections this isn’t an issue, on others I can’t hear the other end of the call. Also, Since I have a team that uses the system the same way, some of them don’t have any issue at all and some of them do.

Is it something to do with NAT? Everything I’ve found online to remedy the situation has been confusing and totally worthless.

If you all have any idea, it would be greatly appreciated.

Need a diagram of your setup.