Applies To

Call2Teams - Customers


In the event of PBX service unavailability, service records (SRV) are used to determine what the PBX failover destination should be (if an SRV record is not available, the A record will be checked to locate the failover destination instead, using the IP address of the PBX and port 5060).

An SRV record has the following format: TTL class SRV priority weight port target.
  • Service: The symbolic name of the desired service e.g. _sip.
  • Proto: The protocol being used e.g., UDP, TCP, TLS
  • Name: The domain name for which this record is valid against (followed by a dot)
  • TTL: Standard DNS Time to live 
  • Class: Standard DNS class field
  • SRV: The type of record it is (in this case it is SRV but it could be a different type of record such as an A record for example)
  • Priority: The priority of the target host (determines the primary and the failover PBX, lower value means the higher the priority/more preferred e.g., 0 being the highest priority in the below example)
  • Weight: Relative weight for records with the same priority (higher value means more preferred)
  • Port e.g., port 5060 for SIP traffic
  • Target: The canonical hostname of the machine providing the service (followed by a dot) 3600 IN SRV 0 5 5060 3600 IN SRV 1 5 5060

The above SRV example states that the first line ( should be tried first due to the priority being set to 0. 

The line beneath is the failover as the priority is set to 1. 0 is considered a higher priority than 1 and 1 would be considered higher than 2 etc. if there were more entries added.

Should the primary PBX (0) become unreachable (if there is no answer from the TCP/UDP connection), the SRV record states that would become the failover PBX and take its role as the primary PBX. The SRV record contains the information required to locate the failover PBX and use it as the desired service (domain name, protocol and port etc.) until such a time as the connection with the primary PBX could be reestablished. 

The Solution

Once the SRV records have been populated in DNS, the SIP domain (and/or SIP proxy) are entered into the Call2Teams portal in the Services > PBX tab. In the event of the associated PBX being unresponsive, Call2Teams will automatically try to look up the SRV records against the given SIP domain (or proxy) to try to reestablish connectivity (failing back to using A records if no SRV records exist against that domain).