We present a specification and implementation of an ISDN-related specification in the formal description technique LOTOS. The purpose of this work has been twofold. Firstly, we confirm the applicability of using LOTOS to specify telephony applications beyond Plain Old Telephony Systems (POTS). Secondly we have semi- automatically generated executable code from our LOTOS specification. This code has been used to run a small telephone exchange, thus demonstrating the potential use of LOTOS as a rapid prototyping language.