Two weeks ago, I bought a used phone (described like this in the order confirmation "Apple iPhone 6s Plus Rose Gold UNLO TECH" from an online dealer in refurbished phones.
I used it without problem with a Vodafone Sim (PAYG). Yesterday I arranged with BT to add my mobile to our telephone and broadband contract, and requested a PAC from Vodafone.
Today I have fitted the BT Sim and the phone shows "No Service".
I rang BT and explained the situation. The BT person said that mobile coverage in our area was not brilliant but I should get 3 bars. He suggested that the phone could be locked.
I contacted the people I bought the phone from, asking them to confirm that it was unlocked (confidently assuming that they would say straightaway that of course it was unlocked). But they have said (this is all by FB Messenger as they do not have a phone) they will send me a postage-paid return label and will investigate.
In a later message they asked if I want a refund or a replacement.
To be honest, I am flabbergasted and don't know whether I should ask for a refund or a replacement – or what I should say I want.