In some two-way communications systems, radio operators have a need to communicate with landline telephone parties, as well as other radio operators. These systems frequently employ unique individual identification (ID) codes for each radio in the system, similar to telephone number extensions in a private auto branch exchange (PABX). In this manner a level of privacy is achieved, as radio units only respond to calls directed at their unit ID.
Responding to calls initiated from landline parties typically requires the radio unit operator to manually send an "access code" signal to the interconnect device at the fixed end to establish the audio interconnection. By requiring a licensed radio unit to initiate the audio interconnection, access to the radio system from unlicensed landline callers can be resticted, thereby meeting the requirements of the Federal Communication Commission Part 90 Rules and Regulations for all applicable radio services. Responding to the radio unit initiated calls does not require this "access code" to be sent, as both parties are licensed operators on the system.
As a matter of convenience it is frequently useful to assign a common ID for receiving radio unit-initiated calls. However, because the radio unit decoder is unable to distinguish the source of the call (landline or another radio unit), the receiving radio cannot give an indication to the operator whether an "access code" needs to be sent to communicate with the caller. Accordingly, a need exists for a method to differentiate between landline-initiated and radio unit-initiated calls, and thereby, to differentiate the call-origin visually and/or audibly to the radio operator. In so doing, the operator is able to manually initiate the necessary "access code" only for landline-initiated incoming calls.