Make sure the handset receiver is out of the cradle, and that there is a green power light on the base, and a green headset light as well.
Double check the way your S12 is plugged in. On the back of the S12 there are 3 jacks. The headset plugs into the jack on the left, the handset receiver plugs into the jack in the middle, and in the right jack there should be a straight black cord that plugs into the “receiver” jack of the actual phone base.
Next check the compatibilty switch. Slide the compatibility switch until you hear a clear dial tone. There may be multiple settings with a dial tone. Select the one that sounds most like the handset of your phone. The most common settings are 1 and 4.