You could install sip droid on an unlocked android like the adp1 (or install unknown sources on a carrier phone) or go with any number of sip clients for pdas.

You could install sip droid on an unlocked android like the adp1 (or install unknown sources on a carrier phone) or go with any number of sip clients for pdas. Then you still need to provision a line for the other end of the sip connection, but you'll have a pda that can make calls.