The FX-5500 is a single set, the FX-5520 a double set. 

If you want to connect the handset of the FX-5500 to the base of the FX-5520, the other (new) base should not be connected to the power, the base of the FX-5520 should be connected to the power, here you can connect the handset of the FX-5500. 

Please see the user manual about registering handsets. 

After registration and when all handsets have received a number from the base of the FX-5520, the base of the FX-5500 can only be connected to the power, to charge the handset.



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