3CX SIP Registration to FQDN A Record with Multiple IPs

3CX SIP Registration to FQDN A Record with Multiple IPs

​• Assuming that under Host field you enter the FQDN that resolves to and (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
​- UDP 
​- 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

• 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. 
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