If your phone does not auto provision when you plug it in there are a few reasons for this but they can be checked easily. The first most simple thing to do is factory reset the phone, on a snom phone unplug it from the power supply and hold the # key when the phone is booting. This will bring you into the recovery menu as below, press 1 to factory reset the phone.
On a Yealink phone this process is even easier simply press and hold the ‘ok’ button on the phone and it will ask you if you want to perform a factory reset as below;
If your phone is still not auto provisioning after these steps please follow the below guide for a Snom phone;
For a yealink phone please refer to this guide;