I don't have this model but I have had two Siemens phone systems. Typically, when the handset says "Searching" it is trying to connect to the base station (the part actually connected to the phone line). This can happen if the handset is too far from the base station to connect to it wirelessly. It can also happen if the handset has become unregistered from the base station (or was never registered to begin with). If the distance issue is not the problem, look in your documentation for how to register the handset and give that a try.
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I don't have this model but I have had two Siemens phone systems. Typically, when the handset says "Searching" it is trying to connect to the base station (the part actually connected to the phone line). This can happen if the handset is too far from the base station to connect to it wirelessly. It can also happen if the handset has become unregistered from the base station (or was never registered to begin with). If the distance issue is not the problem, look in your documentation for how to register the handset and give that a try.