I am not fully sure but if you re-register the gateways to the secondary Callserver that can be done in LD 135
SMGR xx yy Forces Media Gateway to register on a specified Call
Server, where:
• xx = specified Call Server. Options for this parameter are
PRIM (Primary), ALT1 (Alternate Call Server 1), or ALT2
(Alternate Call Server 2).
• yy = cabinet number of the Media Gateway to be
switched.
Note:
The GRDRC ABKUP prompt must be configured as
SCHD on the primary and alternate system for this
command to work
For IP phones you could just shut down TPS in the Sig Servers at the primary site, Command line "disiTPS" the phone setup would normally be to go to the Secondary System First so should re-register. The GRSC should activate the Secondary server when the registered devices go above the threshold set.