from the CAA pdf:Сообщение от Military_upir
... attempt to establish radio communication with the intercepting aircraft or with the appropriate intercept control unit, by making a general call on the emergency frequency 121.50 MHz, giving your identity and the nature of the flight. If no contact has been established and if UHF is fitted, repeat the call on the emergency frequency 243 MHz ...
from FAR/AIM 2004 (5-6-2 3.c.3):
Attempt to establish radio communication with the intercepting aircraft or with the appropriate intercept control unit, by making a general call on the emergency frequency 243.0 MHz and repeat the call on the emergency frequency 121.50 MHz, if practicable, giving the identity and position of the aircraft and the nature of the flight.