I really want to say 'no way'. I just called them because the the alarm handler in Signal can't even bring up the text message in the GCM alarm. A GCM can dial a pager with a numeric message and that's about it. There is some kind of DDE exchange but Signal is so old I don't know if you can get it to cooperate. You can buy a seperate device like people use for flooding or heat failures in their cabins and program an output to go to that device and dial out a voice message. Signal was way ahead of it's time bit that time has passed. Keep us posted what you find.
Second place is the first loser.