Fresh install of SNG7-1904 and having the same problem with Asterisk 13.22 and FreePBX 14.0.13.4.
Restarting Asterisk helps for a few minutes, then the problem returns. This was a box that had settings imported from a 10.13.66 box using the conversion tool. I used the Asterisk CLI to verify all settings both on the extension and transport there don’t seem to be any differences.