I'm having this same problem as the OP had. I checked before porting my landline to Tmobile and then to GV. Originally before I started, I typed in the number at GV, and although it said it was a landline and couldn't port it, it didn't give me any other reason for being unportable.
So I figured once it was ported to Tmobile, which happened yesterday and wasn't a problem, it should be portable to GV. Now that the number is at Tmobile, when I try to port it to GV and type in the #, GV says they can't port it because they don't support numbers in the area.
Could this be happening because I'm still in the dual service mode of my line being ported to Tmobile and that first porting isn't 100% complete yet (I'm still getting calls on that number from both phone services, even though its been about 36 hours since the port started)?
My number is in the 952-856-XXXX range, and when I checked, it said Broadband (the service Google Voice uses) operates exchanges in the 952-856 range and I've read on forums that others actually have a Google Voice # that begins with the same 952-856 area code and prefix.