handsets in multipule groups

When I first purchased Office@Hand, I had set up two ring groups using digital phone numbers. Basically I had the first group (A,B,C) ring for 30 seconds, then the second group (A,B,C,D) ring for 50 seconds.  Office at hand did an update and I lost the ability to add (A,B,C) to the second group by restricting the digital numbers not to be added as an phone number.  I run a small towing operation and I have 3 dispatchers working and if I could add the same phones to multiple ring groups I could help answer phones and still allow them to answer if I see they are getting off the line. 

I understand the digital line was a glitch and used it as a work around, but not sure why we can not clone the phone and put it in multiple ring groups.