which app to play audio file over phone?

I want to edit a special voicemail greeting on my pc, then save it as an mp3 or wav.

Then I want to make a call to my operator (tmobile), select 'record new voicemail' then when it starts quickly switch to an application that will play my mp3 file and send the audio down the phone to tmobile. Then I'll have a nice answerphone greeting, right?

Which app will do this, ie play audio that gets transmitted via the phone call? I've come across call recording software, but not this. Any tips?

Thanks, John