To add local inbound calls from POTS, I pointed my GrandCentral (GC) account at Gizmo. (They do that now.) This was the tricky and annoying part because the console showed that the call had been answered but the calling phone was still ringing.
When I stuck a conference room into the mix and dialed into that with a third phone, I realized that GC was using an IVR on the receiving end (i.e., "Press 1 to accept the call, Press 2 to send it to voicemail, etc.). In other words, it requires human intervention (i.e., you must press 1 to accept a call).
That's not to say that it can't be worked around. You can either have Asterisk push it immediately to a hard phone (if you expect to treat is as it was intended) or you can "trick" the IVR into delivering the call with the SendDTMF command (notes here). In either case, I now have a local inbound number for free!