We present the development and characteristics of a basic ASR system for isolated digits in Spanish, used over the telephone line. Initially we will introduce our first idea, a basic discrete system, and then we will see the improvements we made to increase the recognition rate at a low CPU cost (always considering its practical implementation as a real time system). The most remarkable advances were obtained with: 1) Semicontinuous modelling. It is a more precise modelling, although more time consuming. 2) End-pointing with a Neural network. 3) One pass decoding with noise models. The intention of both 2 and 3 is to alleviate the effects of a wrong end-pointing. 4) Parametrization using perceptual filters in frequency and filtering in the time domain (RASTA-PLP). We wanted to decrease the effect of telephonic noise in our system.