why my friend's iphone 5s is always busy on a first call and is ok on redial?

A friend of mine has an iphone 5s. Since a week back, all incoming calls to that phone on a first dial go busy ( "number busy" appears on the caller's phone) and on an immediate redial, the call is normal. We have tried this from a few different numbers and the problem persists. Is it a problem with the setting etc?

Kindly advise.