On the side of your 816 cabinet, you have modular jacks. Probably 4 below the amp connector for lines and 4 or so above for paging, com 1&2, etc. By default, com1 is used to program the box and com2 is for SMDR. The phone system will give you a printout of all calls incoming and outgoing and I remember it's very forgiving if you turn it on and don't connect anything to it.

You might reach out to a Comdial tech and get a cable and use an old PC to capture the call records.

Carl

This model is end of life