Abstract: A signal processing circuit is provided, one embodiment of which is employed in an inbound telemetry device such as an inbound MIU, another embodiment of which is employed in an outbound telemetry device such as an outbound MIU, for example. The inbound signal processing circuit monitors and senses conditions on a phone line coupled thereto to determine when the line is available or not available for use by the inbound telemetry device. More specifically, the processing circuit includes a sensing circuit which senses when other communications devices coupled to the phone line are off-hook and hence the line is not available. The processing circuit advantageously employs the same sensing circuit to determine when ringing signals are present on the line such that the line is not available.