Sharp UX-BD80 Operation Manual page 47

Additional bases
One cordless handset can be used with up to four bases. By purchasing
additional bases, the range of use of the cordless handset can be expanded.
(Note that once a cordless handset receives a call, it must remain within the
communication range of the base that is transmitting the call.)
♦
A call can only be transferred from a cordless handset to the base that it is
currently using, or to another cordless handset that is using the same base.
♦
To use a cordless handset with additional bases, the cordless handset must
be registered in each of those bases. See page 65.
About the Cordless Handset System
45