- If 4 handsets are already registered, one of the handsets would display Headset 4 * For M66 DUO units (e.g., M6602, M6652) a maximum of 3 handsets can be paired (or registered) with the same base station, please check whether your base is fully registered (If 3 handsets are already registered, one of the handsets would display Headset 3) * If your base is not fully registered, follow the steps below to pair (or register) the handset with the base station * If you are using a non-Philips brand handset or base station, refer to the user manual of that handset or base station Note: Registration using non-Philips brand handset or base station may not be possible