A phase-locked loop circuit has an output amplifier and a main feedback path from the output of the output amplifier. The NDIR gas sensor has an extended optical path to increase precision and accuracy in the measurement, and substantially increased ventilation opening size to facilitate in and out of the target gas through the optical cavity thereby decreasing the response time required for measuring the gas concentration. The temperature control system further has a second member for surrounding a first portion of the thermally conductive wall. A call receiving pager apparatus, system and method which enables two-way text messaging as well as the storage, retrieval, and recall of telephone numbers making calls to the pager apparatus where the recalls are made by way of sending a data package to the stored caller number is described.