A little late, but for posterity...
Weatherman66 wrote:is it possible to answer calls or retrieve messages to the land line number with a cell phone?
No, not with a cell phone. With one of the system's handsets, yes, but not with a cell phone - linked or not.
The phone system's is essentially a Bluetooth headset with respect to your cell phone. Using the system, you can call out on your cell phone, you can accept an incoming cell call, get your cell's ringtone*, and get phonebook entries*.
*if your phone supports these features over Bluetooth
Weatherman66 wrote:How many cell phones can use any of the features of this phone?
Maximum of two phones can be paired to this system. Only one may be in use with the system at any given time. Both stay paired, but only one can be actively in use, like Bluetooth headsets that can multi-pair. Nothing stops you from using the other paired cell phone normally with another Bluetooth headset (you might need to tell your phone to connect to the other headset), or just on the handset.
See page 18 of the manual for full details.