For some reason my mobile phone won't call my dad.
When ever I try to ring him, it rings once and then says "The person you're calling is on another line, please leave a massage after the tone"
My dad is never on another line as I've tried ringing him when I'm right next to him and it just comes straight up with a missed call from me.
He can ring me no problem though.
Who's phone is the problem and how do I sort it?
Thanks in advance if anyone can help!

Try doing a full battery out reboot on your phone.
Also check to make sure there aren't any redirects set on your dads phone.