Hi @robertsbill03,
It's certainly possible this could be genuine, as we do use 03 numbers in that format. Unfortunately "spoofing" is more and more prevalent, which is where scammers fake the number they're calling from. Most give themselves away as they call from 01, 02, or 07 numbers which wouldn't be used by organisations like Three as those are personal landlines and mobiles.
The only way to be 100% certain is to get in touch, and customer services will have notes on your account if we've been trying to reach you.
You may also find this page on fraud to watch out for helpful, as it has tips about various scams and how to report them.
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