- CD18 and CD19 series do not offer intercom function. Therefore registering a handset would not offer intercom function. * If you are using a non-Philips brand handset or base station, refer to the user manual of that handset or base station * Registration using non-Philips brand handset or base station may not be possible. * A maximum of 4 handsets can be paired (or registered) with the same base station, please check whether your base is fully registered (If 4 handsets are already registered, one of the handsets would display ?Handset 4?). * If your base is not fully registered, follow the steps below to pair (or register) the handset with the base station