Hi gerdanger... You might want to elaborate more...
Do you mean that whenever you call your friend's number his phone would ring on your end but he denies that his phone would ring whenever you attempt to call him?
To validate this you may want to call his number when hi is beside you and check if his claim is true.
Have your friend check if call forwarding is active on his end. He can deactivate it under phone settings (it varies from one device to another).