问题：

[多选] Which two VoIP capabilities are utilized when the CallManager and the gateway are configured for Survivable Remote Site Telephony？（）

A .  Analog phones， connected to the PSTN， act as internal call agents if IP communication to the CallManager is lost.
B .  An onsite gateway reroutes calls over the PSTN if IP communication to the CallManager is lost.
C .  The IP phones can reroute calls over the PSTN if IP communication to the CallManager is lost.
D .  Local voice gateways can assume the function of a call agent if IP communication to the CallManager is lost.
E .  IP phones use an internal call agent if IP communication to the CallManager is lost.