Assuming that you are using registration for your Telnux trunks, disable source port rewriting; see
and set the firewall optimization to conservative.
If using IP auth, allow all addresses from which Telnyx can send calls in Match (Permit) and firewall rules: Whitelisting Telnyx IP Addresses | Telnyx Support.
If you still have trouble, consider using SIP over TLS to prevent the firewall from messing with the packets.