I have a common situation: each of several classrooms has one phone and two teachers. Only one teacher can be assigned to the phone, the other has just voicemail. We need the ability to assign the phone to two teachers each with their individual extensions. Then there should be the ability to assign distinctive ringtones to each extension. This is not the same as hot-desking. I cannot require teachers to login and logoff their phone. This should be possible with multi-line phones as are common, such as the YeaLink T-23G.