This paper gives a very brief overview of a formal specification of th
e behaviour of a set of about 10 realistic features for ISDN telephone
s, The full specification employs a multiparadigm technique in which p
artial specifications in different languages are composed, but the foc
us here is on the use of the Z notation to specify call processing and
subscriber database aspects of telephone features, Our experiences wi
th the approach, and with Z in particular, are discussed.