The TUUM-4, multi-frequency transceiver, with continuous monitoring of the NATO frequency can be used with pre-programmed channels or user-defined. Its modular design, combined with mechanical robustness and exceptional reliability, make it the ideal telephone both for surface ships and submarines. When configured for surface use, omnidirectional transducers are preferable. When configured for underwater use, it can be combined with sets of directional transducers for increased range and discretion. The TUUM-4 comprises an ER 4A/U transceiver and associated transducers. This equipment is compatible with underwater telephones and most transducers already in service. As with other means of external communications (UHF, VHF radios ...), the TUUM-4 can be Interfaced with the ship's internal communication system.