Answer phone. I tend to leave my answering machine on all the time to see who is calling as we get

Question

I tend to leave my answering machine on all the time to see who is calling as we get a lot of sales calls. However, if i know someone is calling the phone doesnt ring for long enough before kicking into the answer machine. Does anyone know a way that i can make the ring last longer while it is set on answer phone?Thanx