Caller ID on a Presence Button

We are using Mitel 6940 phones. When a Employee's number rings, it would be great for his/her assistant to see a caller ID on a line added with a presence key. They can currently answer but it is not obvious that the presence line is ringing. We think a Caller ID on a presence line would make the fact that it is ringing more obvious, and more importantly the assistant knows who is calling his/her boss and can answer if needed.