Mtn blocking a number that does not owe.

My sister has 2 contracts under her name. One has my dad's number which is up to date with the payments and the other one is for a contract she took beginning of the year. So my dad's bill is up to date with the payments but the other one that she uses for the router is in arrears. So what I don't understand is why mtn has blocked the number with the installments that are properly paid and not the one that is in arrears.