I know you can with T-Mobile, as I am doing this.  Each of the four
phones we have on our Family Plan are different models. T-Mobile gives
unlimited T-Mobile to T-Moblie minutes to people on a family plan, or
to people that add Unlimited T-Mobile to T-Mobile minutes to their
single line plan.  AFAIK, other carriers have simular plans and
procedures.  I haven't really researched it though.

FTR, one of the lines on our family plan is even in another state.
This was no problem as long as we had all the phones numbers based on
the same locality.  So all the numbers are local for one location.
This is an inconvienence for the one person out-of-state, but since
most everyone has a cell phone (and long distance is basically free
from a cell phone except for airtime used), it is not a big problem for
others not using T-mobile to call this person. It was a way to give one
of our children a phone for a low monthly cost to us (so we could keep
in touch).

Somewhere around Sun, 12 Mar 2006 20:35:36 -0600, while reading
alt.cellular.cingular, I think I thought I saw this post from Michelle102196
<Michelle102196.24l7cz@nospam.cellphoneforums.net>:

You can do it with any of the companies, but t-mobile is probably the
cheapest.