There are various types of failover, including: 

SIP to SIP Failover 

If your IP PBX (hosted phone system) or SIP endpoints (such as an IP phone) are unable to receive calls, your provider can use SIP to SIP failover to reroute the call to the backup SIP destination of your choice.

SIP with PSTN Failover

If Internet downtime causes your SIP device to miss an incoming toll free or local call, your SIP provider’s network can forward the call to a PSTN phone number of your choice.

PSTN to PSTN Failover

Having problems receiving calls through the PSTN? Ask your provider to automatically forward those missed calls to alternate PSTN numbers.

Failover to SIP and PSTN

This option is for those who believe that it’s always better to be safe than sorry. If you are worried about missing toll free calls, your provider can configure a variety of failover options to both SIP devices and PSTN phone numbers.