• Assuming that under Host field you enter the FQDN that resolves to 1.1.1.1 and 2.2.2.2 (A, AAAA, SRV or NAPTR - does not matter).
• Assuming that under Sip Trunk Options, you have selected under Transport the option ANY
• The PBX, after resolving the above FQDN, will generate the following DNS table for the Voip Provider
- 1.1.1.1 UDP
- 1.1.1.1 TCP
- 2.2.2.2 UDP
- 2.2.2.2 TCP
• The first one in the list will be used
• Upon failed registration (if registration based) and/or 503 error message from the Voip Provider and/or request time out (from the PBX to the Voip Provider), the first candidate will be greylisted and the system will use the next candidate/IP
• When all candidates are used, the PBX will forward a new DNS request for the configured FQDN
NOTE:
• If the IP Mode (sip trunk > Options) is also set to Any, IPv6 records will also be added to the system IP candidate list
• If the UAS does not accept TCP transactions, you may select UDP transport only, so the candidate IPs are limited to the accepted ones.