I have a Moto e4 plus my out going call disconnects automatically. I'm facing this issue only with one number. Also the person receives a missed call notification.
Bring the phone into your carrier and ask if they can assist you. If not, bring it into his carrier and ask if they can assist you. (It would help if he went with you with his phone both times.) It can be a problem with his phone, but it sounds more like a carrier problem on one end or the other.