See ARS
If a primary trunk is unavailable, then Alternate Route Selection (ARS) provides automatic fallback to an available trunk, for example, analog trunk fallback if a T1 or SIP trunk fails, or use PSTN for SCN fallback.
By configuring ARS, the system can route calls through the optimum carrier. You can also use time profiles to take advantage of less expensive rates or better quality at specific times of day.
Multiple carriers are supported. For example, local calls are to go through one carrier between specific hours and international calls through an alternative carrier. Carrier selection using two-stage call set up through in-band DTMF is possible. It is possible to assign specific routes on a per user basis to only allow expensive routes to be used by critical staff.