They are definitely scammers. They got my number and in 2018 somehow debited some money from my credit card and the money went to an account in Lagos, Nigeria. Since then they call periodically using all sorts of different numbers, some looking like an Australian. number, some like this one from London and some with other countries prefixes eg.g Estonia. I usually block them but occasionally pick up the phone before realising it is them. Even when I am nasty to them and tell them they are wasting their time, they still call. They keep telling me that they are holding some money of mine and want to give it back to me...yeah sure! Anyway NEVER believe them and hang up as soon as you can.