It depends why the voice service is down. If the voice service is down because the only cell tower in range just got hit by a giant meteorite, then obviously, your SMS isn't going to work, either. If you can't get through by voice because some big event is happening and everyone is trying to call everyone at once, overloading the system, then SMS could very well get through when voice does not. SMS is carried at the signalling level and uses much less bandwidth than voice, so it's easy to prioritize.
If "voice service is down" means "calls start OK but keep getting dropped quickly", then it's very likely SMS will make it through.
John C. Fowler, snipped-for-privacy@yahoo.com