None at all, used them in conjunction with my BT line for many years, they are a good company in my experience. They have (or at least used to have) a number of different brand names. I trust them sufficiently that I haven't bothered to cancel my account and direct debit with them even though I haven't used them for nearly a year.
I have not used them in conjunction with a mobile but I am just about to give it a try.
The only problem is that some phone companies try and block these freephone numbers and/or charge for them, if you read your very small print, somewhere there is usually a clause about calls to redirector companies.