Compaq Armada 7400 Reference Manual page 170

items that can be connected in parallel without impairing the
performance of the standard bell circuit.
REN = 3/n where n is the maximum number of units that
may be used in parallel.
The maximum REN of a line is 4; therefore, the total REN
of all apparatus connected to the line (obtained by summing
the individual REN values) should not exceed 4. This value
includes any BT-provided instrument, each of which is
assumed to have REN value of 1.0 unless otherwise stated.
Exceeding the REN 4 = 4 value may cause your
telephone(s) to ring weakly or not ring at all.
Only one modem should be connected to a telephone line.
Connecting a modem in parallel with one or more telephone
instruments is not allowed.
The REN for this piece of equipment is 1.0.
The exchange should provide facilities for either loop
disconnect (pulse) dialing or multifrequency (tone) dialing.
The modem can be configured to operate with either system.
During dialing, this apparatus may tinkle the bells of other
telephones using the same line. This is not a fault, and we
advise you not to call the Fault Repair Service.
The Compaq Armada 7000 Series II (Series 577) modem
specified in this guide is for the Compaq Armada 7400 Family
of Personal Computers.