Doesn't transfer tie up two sessions if Voip to Voip or your PSTN line and one Voip session?
Yes, by default it bridges regardless of the service provider.
Some of the information in the manual leads one to believe that the bridging can be avoided if both calls are Voip and your ITSP supports the transfer function but I'm not sure what providers might support it.
That's the SIP Refer mechanism, but recent posts seem to indicate there may be problems with the OBi's implementation of it.