What Happens To Sim Numbers Of Deceased People?

i decided to ask this question after certain events.

So i decided to call my late dads mobile number (its mtn tho) out if plain curiosity, I was surprised it rang n sumone picked it, i immediately established friendship wit this individual who intimated me on how he bought d number from mtn office. He explained dat immediately he inserted d number, calls started coming n even bank alerts...

Mind u, my late dad used dis number for more dan 10years n barely 2years after his death his number was sold to a new customer.

my questions:
the
1. Is it legal for mtn to do dis? (make i sue na na)

2. is dia a procedure for deactivating sim numbers? (i can imagine if dis sim had fallen into d hands of fraudsters)

my nairaland family pls advice..