Same is now happening to me too, but from 0800 052 1251 - have had a call on Friday, Monday and this morning while I'm at work.
I answered the one yesterday, to a woman who refused to take no for an answer - I mentioned three times that I wasn't able to discuss my contract right now and had to be in a meeting, to be met with 'don't worry, it will only take a couple of minutes' and 'don't worry, I'm sure you have suitable time now', before she finally took the hint and the call ended. Only to be rung again today, again while in a meeting.
Even better, having checked the marketing consent preferences for my account, the 'sounds good, keep me up to date!' box is unticked. So not entirely sure why they're even phoning at all.
And they're phoning my mobile, despite the contact phone number for my account being our home phone (not that I want them phoning that either, as then they'd just be interrupting my wife working from home).