Older thread, but I hope this will help someone. Walmart, AT&T, and Verizon sell cellular to RJ-11 adapters. They're basically cell phones and have an analog RJ-11 jack on them. I know these have been around for years in various forms, as we had a few of them on our satellite trucks which had AT&T Partner PBX/Key systems on them. They provide a regular dial tone just like you (and your PBX) are used to.

Verizon Home Connect: http://www.verizonwireless.com/wcms/...ome-phone.html

AT&T Wireless Home Phone: http://www.att.com/shop/wireless/dev...homephone.html

Straight Talk Wireless Home Phone: http://www.walmart.com/ip/Straight-T...Phone/22084643