The V.8 specification defines a procedure to be used as PSTN modem answer phone calls, which allows the modems to negotiate the optimum modem standard, which both ends can support.
At startup the modems communicate using the V.21 standard at 300 bits/second. They exchange simple messages about their capabilities, and choose the modem standard they will use for data communication. The V.8 protocol then terminates, and the modems being negotiating and training with their chosen modem standard.
