Voice Services -> SP2 Service -> X_InboundCallRoute: sp1(18005551212)
Change 18005551212 to your cell number.
All inbound calls on SP2 will be routed to your cell via SP1 trunk.
Use OBi expert to make this change.
Callcentric will do what you want.
You can setup 4 extensions then define a call Call Treatment to hunt for a free extension.
Your OBi100 can only register to 2 extensions and only has 1 phone port.
An OBi202 can register to 4 extensions, but only has 2 phone ports.
You can use your OBi100 for a 3rd phone port.
In addition to the OBi202 you need to get another ATA to get the 4th phone port.