Bit Rate and Information Rate
There is a distinction between these two ideas. The bit rate is the advertised bit rate of a communications channel. In the case of an ISDN B channel in Europe it is 64 000 bits per second.
Information rate is the rate at which information is passed over the channel. The information rate can never be higher than the bit rate, but it might be lower.
An ISDN telephone transmits and receives 64 000 bits per second, even when there is total silence.