Thanks Taoman and drgeoff, I see that this will probably become too much of an "academic exercise".
My current SP1 SIP provider (Freephoneline/FPL) does have a CF option but it wasn't working consistently, hence my search for an Obi solution. Plus I was becoming more comfortable with the Obi and wanted to explore the possibilities. I want to keep my FPL because it's free in/out across Canada and it's been reliable for years.
Adding FPL to its Inbound Call Route works consistently better than FPL's own service but uses both available sessions. Increasing the max sessions didn't play well. It seems FPL doesn't allow more than 2 sessions. I tried SP2 = GV in the Inbound Call Route and it treated the call as a Call Forward after 1 ring on home phone. Using SP3 =
voip.ms in the Inbound Call Route worked as well as FPL. But I don't want to move my DID to
voip.ms and/or increase my cost. After all, that was the reason for employing the Obi.
That's when I started considering how Voice Gateways might help. I just needed a gateway to call my cell/sip client. The Obi guide and
voip.ms Wiki described the use of SIP URI calling which I used in the past. If this worked I could make an informed decision on solution re:"cost/benefit". But this seems to lead further down the rabbit hole (e.g. creating another - external - SIP account etc)?