 In this system the two tones for any one side of the connection are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase.Voiceband modems generally remained at 300 and 1,200 bit/s (V.21 and V.22) into the mid-1980s.A V.22bis 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212 signaling was introduced in the U.S., and a slightly different one in Europe.