Back in time phone lines often failed in Mexico. People would say ¡bueno! (Good!) to test the sound quality, and it became a standardized protocol to establish communication over the phone. Over the years phone lines have been improved, however, the usage of “bueno” remains. Other words which can be used to answer the phone are:
¡Sí! — ‘Yes’
¡Diga! — Say! (Used as a command)
¡Hola! — ‘Hello’ (Not very common)