Firewall Rules for FreePBX on Telnyx

Define group SIP as UDP and TCP on Ports 5060 and 5061
Define group RTP as UDP on Ports 16384 through 32768
Define group TelnyxSIP as IP addresses and
Define group TelnyxRTP as network ranges:
Define FreePBX as IP address <internal LAN interface of your FreePBX>

Create your Inbound Allow Rules:
Allow SIP from TelnyxSIP to FreePBX
Allow RTP from TelnyxRTP to FreePBX

More details can be found here: