There are two options. With tone decode you could program a given frequency to only receive when a given tone is encoded. Thus, other transmissions using a different tone would not be heard. The other option is the tone reject card which would not allow transmissions with a given tone to be heard, but would receive all others. Tone decode is the most common, but I have had circumstances where tone reject was useful. It is worth noting that CTCSS is decoded at the audio level. Thus the unwanted signal is being received, but there is no audio heard. If two signals on the same frequency, but with different tones are both present, they will be heard resulting in distorted audio or one will capture out the other. CTCSS is not a magic bullet. Fundamentally, it is used to only allow properly encoded signals to access a repeater. Where repeaters have CTCSS on their outputs (not all do and some use a different tone on the output), then it is possible to allow sharing of channels provided one signal is stronger than the other.
I suspect that it will work for you. The entities using MED 1 would hardly tolerate significant interference for routine communications.
Last, are you sure these users are using CTCSS? They could be using DCS. Before you order tone boards make sure that they will work with the systems you want to monitor.
