From the ring group, there is an "overflow point" field that you can fill out with another ring group or hunt group, a valid extension can be put there. After the timer expires on the ring group it flows into the overflow point. If there isn't one, then it should "Ring until answered". You can set up 2 ring groups and bounce the call between the 2 until some one answers. (I am tempted to do this in my facilities since the end users despise answering a phone, and find it convenient to blame my beloved Mitel)