FXS channel number

How do I determine the channel number when defining an FXS extension?

The TDM410 seems to be installed properly. It’s reported by dahdi_hardware and dahdi_tool shows the phone going off hook. But I’m stumped on setting the channel number under “Add DAHDI Extension”.

On the TDM410, Port1&2 = FXO and Port3&4 = FXS.

Since this is the only analog card, I’m guessing that port 4 corresponds to channel 4? But I don’t get dialtone after configuring the extension using channel 4, so I want to confirm the channel number.