Well... a ring group has the ability to send a caller somewhere if none of the members answer after a given time. Unfortunately, when you throw in mobile phones, it can complicate matters. First off, you pretty much have to using VoIP trunks to sent the calls to the mobiles so that correct answer supervision is returned when one is answered. In addition, if any of the mobiles has a Voicemail option (Mobile operator Voicemail), then a call answered by that will count as an answered call. this will happen if one of the mobiles is currently on a call, or, out of the area/unavailable.
When you build the ring group and want the calls to ring external numbers, the members must be PBX extensions that are set to ring my mobile. I don't believe that extension call forwarding will work on a ring group call, at least, it never used to.
You would have to do some testing to see if this will work reliably. As soon as you start sending calls externally, you can run into situations that you don't have much control over.
