Unlocking Samsung J5

Hi,
My sister has given me her old Samsung j5 phone but i think it's locked to Vodafone (her network provider) and I'm on o2 PAYG. When I put my SIM card in first it went through the phone set up, got to point to select network and I selected o2 PAYG, all seemed fine but when trying to make a call it says 'Not registered on network'. I've tried to go through settings and selecting network, it tried for ages then says not available try again later.

I've been into local phone unlocking shops but being quoted £25-30 and told will take 4-7 days to unlock! Is there a way I can do it myself?

I have been told she can contact Vodafone to unlock but the phone she gave me she thought she had lost a few months ago, reported it to Vodafone and got a new phone, she later found the original phone. They didn't put a complete block on the old phone as I can use it for everything except calling / texting. If she goes to Vodafone now and says she found old phone and wants it unlocking to another network could they do a complete block / charge her for getting the replacement?

Thank you