These is an expansion handset and will not work without one of the following units.
TRU8866, TRU8860, TRU8865, TRU8880, TRU8885, TRU8888, ELBT595, ELBT585, ELT560, CLX485, CLX465
These would also work if you phone model is one of the above but has a -2 , -3 or -4 in the end.
Example: TRU9485-3