SIP extension over WAN dropping calls

Hello. This is my first post, so I hope I have this in the correct category.

We are having issues with a SIP extension on a WAN losing the connection to the FreePBX server. It happens on internal extension calls and outgoing calls. (LAN extensions work great.)

We just newly installed FreePBX 13, but we have had this problem on an older version first. We have also tried the PBX server and SIP extension at different locations with different routers… still happens.

Currently, we run the following:
Asterisk 13.5.0
FreePBX 13.0.51

The SIP extension phone is a Panasonic KX-TGP500B04.

The problems are as follows:
- Sometimes the connection just drops.
- Sometimes one direction stops working, but the other can hear fine.
- Sometimes the connection becomes very choppy. The voice sounds mechanized… then eventually drops.

The only common denominators I can see are the SIP settings in the Panasonic phone on the WAN and the fact we are using default settings for SIP configuration in FreePBX.

Does anyone have any suggestions of what configuration settings to modify?

Thank you.