Depends. Lots of Variables
Agencies tend to have in their radios
a. The National 700/800 Mutual Aid channels
b. Their Own Off-TRS conventional channels
and if it's only Software failing to connect to the Main Server there is
a. Fail Soft (Where the TRS Repeaters default to CSQ Conventional channels)
and the users are automatically assigned the new Freq based on TG (Pre-determined)
b. Site Trunking, where the Zone is it's own little island, units can still Trunk, but not talk to anyone who is beyond that Zone/Site
What frequencies would be used if 800 mhz system fails?