if you use a simple bridge command with ignore_early_media=true, I think the caller will still hear the FreeSWITCH'es ringback instead of the GSM unavailability message. So, in this case, the caller would simply hang up after waiting for too long.
You can also configure a group call with answer confirmation: the called party would have to press a digit before the call is connected with the caller.
Also you can do some scripting and process the outbound call before you bridge it with the inbound call. Here I have an example in Perl:
Also currently I'm learning how to do that with ESL and Golang.