An OBi can talk to another OBi over the OBiTalk network.
An OBi can talk to a cell phone with a dummy SIP trunk defined on an SPn.
You would have to install a softphone on the cell and register it to a free service like Sip2Sip or Callcentric.
Why would you go thru a 2nd OBi to get to the cell?
You could go directly to the cell with the first OBi.