Simultaneous Ring - race?

Is it common in simultaneous ring implementations to simply use the first line that gives a call-complete indication and drop the others?
I use sim-ring off my PBX at work to ring my cell/desk. If cell is turned off, cell's voice mail answers on the first ring and that's an issue.

Yes, that's standard. To get around this, disable "Auto Answer" on it and when answered, it'll ask the answerer to "Press 1 to Accept". That's what that's designed for since voicemail won't be able to interact with it to accept the call.