There's a way to make the number people use to reach you independent of a specific phone.
Many land line providers offer a service called "group ringing" or "group hunting".
Basically, you give out a number for people to call. Calls to that number route to a list of phone numbers to reach you. You progam the list of numbers: your cell phone of the week, office phone, home phone, etc.
You answer the call on whatever phone you are closet to at the time. The person calling you doesn't notice anything different other than maybe one or two extra rings. Some variations on this feature allow the ability to roll back to voice mail if no one answers the call.
The individual phone numbers aren't visible to those people that are calling you, all you ever give out is the master phone number.
There are two variations to the service, simultaneous ringing or sequential ringing. Basically, do all phones ring at the same time, or ring in sequence.