I am having issues with my Asterisk server when trying to call while on the WAN.
I have no issues calling extensions when I am connected to the LAN the SIP server is running on, but when I take the same phone but try and use its mobile data to call, as soon as I pickup the phone on the LAN gets the hold music.
I have setup my firewall to DNAT UDP 5060 and UDP 10000-20000 to the asterisk server, and configured the firewall on the server to all the same ports.
Before guessing at a cause, you need to provide more info. Set verbosity to 10 and initiate a call from the WAN side. That will show why the phone on the LAN side gets MOH. Once that's explained, turn on SIP debug and you will see if NAT is an issue or something else is going on.
Most likely a NAT issue is causing call to disconnect, but your dialplan grabs the "h" extension and moves to MOH or similar.