A multi-line key telephone system not having a key service unit includes three analog telephone lines to which up to eight, three-line telephones may be connected to provide enhanced service features with cost-effectiveness. System reliability is maintained, notwithstanding the power dependency of the telephones, by including POTS telephones to provide uninterrupted essential telephone service in the event of a power outage. Both data and voice communications between two or more of the three-line telephones is achieved by converting an input digital bit stream from either a digital or analog source into a corresponding out-of-band binary FSK radio frequency output signal that is transceivingly communicated between the three-line telephones as data packets within a TDM frame. The out-of-band signal thus permits analog voice communications on the same line to proceed without interference. A telephone initiating a communications session acts as a master station, establishing inter-station communications synchronous at the frame level and asynchronous at the packet level. The called telephone functions as a slave station in a second time slot synchronized to a first time slot in the TDM frame received from the master. Temporary storage of data in firmware buffers and controlled timing at both the master and slave permits these stations to function within their respective TDM frame time slots.