You can have this happen with a number of phones (not necessarily a no-brainer) a couple that I have seen do this:
(eg HP iPAQ hw6940, Nokia E61i, ...)
- they act as a cellphone when not connected to a wireless network.
- when in range and registered with your wireless network, they act as a SIP based phone which can hook in with any IP-PBX (IP based phone system) that supports SIP based phones on the same network. This SIP enabled phone system can allow you to call across analog/digital lines, acting as a gateway to your PSTN (land lines).
Certainly there are several vendors that you can integrate to do this. For small business, there are a number of vendors that might be worth looking at. An open-source one is the Asterisk server. For medium enterprise I would also evaluate Mitel, Nortel, Centrepointe Technologies, Broadcom, Sylantro, Cisco, Avaya, Intertel, ... lots of others. Check the web for ones that support your phone of choice.