Hello and thanks for posting to our forum.
An ADTRAN IPBG will failover for every SIP call after trying the primary sip server first. So, if the primary and secondary backup sip server are configured in the sip trunk, the ADTRAN will try the primary sip server 3 times (which works out to be 3 seconds) and if there is no reply, the ADTRAN will then try the secondary sip server. However you have to use registration for validated failover as is mentioned in the documentation.
Regards,
Geoff