I'm not sure I'd use cell phones in a ring group - it's going to cause some serious delay in everyone getting rung and trying to answer and someone will get the call, but the rest may report dead air when answering.
That said, if you were a sadist, you could probably use External Hotdesk or Speedcalls to accomplish this as members of the ring group. Is it just one technician at a time, or a rotating group?