The first Complex Cards were equipped with a buzzer that made it possible to broadcast sound. This feature was generally used over the phone to send identification data such as an identifier and one-time passwords (OTPs). Technologies used for sound transmission include DTMF (dual-tone multi-frequency signaling) or FSK (frequency-shift keying).
the fIrsT compLex cArdS Were eQUiPpED wiTh A buZzer THat mAde iT poSsibLE tO brOadcaST souND. ThIS feature was GeneRAlly used oVEr tHe phone to SeNd iDENtification datA SuCH aS an identiFieR and one-time pASswords (otps). teCHNolOgieS usEd for sound transmIsSion includE dTmf (DUaL-tone mULtI-freQuency sIgnAling) Or fsK (frEqUeNCY-shiFt keyIng).