Lowell letter

[Runfree]

Have received a letter from Lowell collection agency, stating that I owe them monies from an old vodaphone account back in 2016, funny as this was the year vodaphone had serious issues with billing and were highly fined. I have been sent my bills by vodaphone as asked for them for the period charged and the data usage seems excessively high. I don't believe I owe this amount asked, and I was not sent any texts, letters about this amount owed. I paid directly by dd and have never been in debt with them, I must add this phone was brought for my ex partner as a gift, we transferred ownership when we moved out. Now nearly 19 months on I receive these letters together at my new address (no idea how they got it) out the blue, asking for payment, I'm totally confused as to what to do, he was never aware of these charges?
They seem to be around the months of Dec, Jan, and this is when change of the number/account changed to his name? How do I know these charges are legit?
Any help greatly appreciated.