Abstract: A method for adaptively switching between the WS and S authentication schemes is based on the customer's usage. A first preferred embodiment provides a 2n-1 state automaton (where n>1), which switches from the WS Scheme to the S Scheme when the automaton passes from state n-1 to state n, and vice versa. At each new registration, the automaton increases one state if more than 1 call was made (i.e., incoming or outgoing) during the cycle; decreases one state if no calls were made during the cycle; and remains the same if 1 call was made during the cycle. The value of n is preferably determined by the PSP based on customer usage. A second preferred embodiment begins each new cycle with the WS Scheme, and will switch to the S Scheme if a call is made during the registration.