Call keys are used to make receive calls if you do not have a DT key for a specific trunk.But they won't light up on an incoming call unless your phone rings.They also are used like an intercom key .Whenever you call another extension they will light up with that call to allow you to put it on hold if necessary.To make any internal calls you have to have at least 1 Call key on the telephone.Multiple Call keys allow you to have multiple calls at the same time.If you use DT keys for outside lines they will not use a Call key but instead light up the DT key.Also please note that DT keys are assigned per line ie DT701 is a line 1 key,DT702 is a line 2 key etc.