1. A method for controlling state transitions in response to a plurality of signals in a voice and data telecommunication system having a plurality of system states and with control being performed by a computer executing a program written in a nonprocedural language and said program controls said system to provide a plurality of features each providing specific voice and data operations and each feature implemented by a set of groups of instructions with each group of instructions being actuated by a predefined one of said signals and said system being in a predefined one of said sytem states, comprising the steps of: initiating a change from the present one of said system states to another one of said system states by the execution of an instruction of a first group of instructions; generating a leave signal to define preparation to exit from said present one of said states; performing common exit operations terminating said present one of said system states before leaving said present one of said system states by the execution of a second group of instructions in response to said leave signal and said system being in said present one of said states; generating an enter signal to define preparation to enter into said other one of said states; and performing common entrance operations to start said other one of said system states upon entering said other one of said system states by a third group of instructions actuated by said enter signal and said system being in said other one of said system states.