I have had this number call me several times, often leaving blank voicemails, but from what i have read on these posts, it cirtainly does appear to be RBS.
I know that Mint and Natwest are part of the RBS group, and according to Tesco Credit Cards Terms and conditions, they seem to be part of this group also. So odds are if anyone has arrears which anything to do with tesco, natwest or mint, then you shall probably recieve a call from them. I am in arrears with my Natwest Credit Card and i am getting calls from them.
I agree that it is annoying, especially as its often a blank call and the voicemails are silent which result in the cost of trying to recieve your voicemail. I wouldnt say its worth reporting them because however annoying as it may seem RBS are entitled to try and recover their money.
If you guys do not owe anything then its possible that it may be a recovery agency who manage RBS arrears along with others. I advise that you ignore all numbers you do not recognise as you are not obligated to answer all calls, and only respond to mail. That way if they say that they have called, you can simply say "if you did then why did you not leave a voicemail with a contact number".