In 1944 Claude Shannon introduce a formula to determine the theoretical highest data rate for a a channel
C=B log 2(1+S/n)
In this formula, B is the bandwidth of the channel. S/N is the signal to noice ratio and C is the capacity (Shannon Capacity) in BPS.
We can calculate the theoretical highest bit rate of regular telephone.
A Telephone line normally has a bandwidth of 3000Hz. The signal to noise ratio is usually 3162-(35db) for this channel capacity is
C=B log 2 (1+S/N)
C=3000 log2 ((1+3162)
C=3000 log2 (3163)