My dad's phone also does the same, and we are both on Virgin Media.
It has possibly been pre-set on the phone line itself.
My dad wasn't able to phone me as we had anonymous call barring (stops people getting through if they withold their number).
There is a number that you can dial if your phone is set to automatically withold its number - I'm not sure but I think that if you dial 1470 before the phone number, then your dad should get through and his number shouldn't be witheld. So, for example, if your phone number is 123456, your dad simply picks the receiver up and dials 1470123456 and hopefully he'll get through and his number won't be witheld.
I'd ring my dad and ask him but he'll probably be in bed now !!!