I opened my AT&T bill to find the same charge. The charge was from HBS Billing Services/United Tel LLC (866) 877-1160. I called that number and got no answer so I called AT&T who put a 3rd party block on my phone for the future and gave me another number to call (900) 687-5298.
The AT&T employee did say they've received other complaints and that the 1160 number appears to be sporadically answered so AT&T found the 5298 number and are giving it out to people.
I called and said I never authorized a charge, never visited their diet site and wanted a refund asap. The guy then said well it's signed up in your name (heh, I hadn't given him my name!). So I said what's the name...no reply. HAH! Again I said you people had no authorization from me to make that charge, remove it immediately, I'm filing a complaint with the FTC, and my state AG's office.
The "credit" is supposed to appear in 1-2 billing cycles.
It all seems like a con game, sick.