We need more information about how the call is created. Are these inbound calls? For inbound calls, you could transfer the call to a queue and have the queue intro prompt play the desired message. If these are outbound calls generated by a dialer, you could have the dialer 3CX Power Dialer play the message then transfer the call to the agent.
We have a tool 3CX Recording Beep that can barge into a call an play a "beep" or pre-recorded message, only once, or at predefined intervals.