In this complaint at the FCC, the carrier lost even though they used a third-party verification system. The reason: vague wording.
The FCC reviews slamming complaints all the time. Section 258 of the TA96 prohibits the practice of “slamming,” the submission or execution of an unauthorized change in a subscriber’s selection of a provider of telephone exchange service or telephone toll service. These complaints are treated seriously at the FCC.
"ISI [the carrier] states that authorization was received and confirmed through TPV (third party verification). We have reviewed the TPV that ISI filed with its response. ISI’s verifier stated that the purpose of the recorded conversation was to “verify and confirm your account information.” However, the purpose of a TPV recording is to verify a subscriber’s intent to change their preferred carrier. As we emphasized in the Fourth Report and Order, “any description of the carrier change transaction…shall not be misleading.”
"ISI must forward to Verizon an amount equal to 150% of all charges paid by the subscriber to ISI along with copies of any telephone bills issued from ISI to the Complainant. Within ten days of receipt of this amount, Verizon shall provide a refund or credit to Complainant in the amount of 50% of all charges paid by Complainant to ISI. Complainant has the option of asking Verizon to re-rate ISI charges based on Verizon rates and, on behalf of Complainant, seek from ISI, any re-rated amount exceeding 50% of all charges paid by Complainant to ISI. Verizon must also send a notice to the Commission, referencing this Order, stating that is has given a refund or credit to Complainant."
So if you are offering Voice (or VoIP) service be certain that your LOA is clear.