 There are provisions for two independent two-way voice channels and the data channel (similar to ISDN BRI) going to a single telephone set.Because each channel is two-way, the two voice channels are rarely used simultaneously, except for example, when a voice "paging" announcement comes over the speakerphone while somebody is using the line, or if an additional "multi-function-module" accessory is attached to the phone that permits the second voice channel to be used as a normal analog telephone line, such as for a modem or fax machine.Several models of MLX telephones are in widespread operation today, the principal difference being the number of programmable "feature" buttons on the phone.