The manual for the Simon XT suggests that there is a cord in the box that can be used to connect the Simon XT to your house phone jack. There is a cord but in reality most people have a standard RJ11 jack and not an RJ45 jack for their phone connection inside the home.
That being said, it is not recommended to connect the Simon XT to a single telephone jack unless it is the ONLY phone being used in the home, because it does not provide "line seizure'. Line seizure means that when the alarm needs to call out in an emergency, it can actually disconnect all of the other house phones, wait for a dial tone, then call out to you or the number(s) you have programmed.
So what they provide in the box is an RJ31X (beige box) and a cord that has bigger (RJ45) ends on it.
Connect the included cord from the RJ31X jack, to the Simon XT LINE connector.
If you don't care about line seizure and want to plug the Simon XT in to a regular phone jack, run down to your local dollar store or electronics shop and pick up a standard RJ11 telephone cord. You may have one kicking around from a phone you don't use any more. Cut one end off and look for the red and green wires inside. You will find that you can (carefully) plug it in to the big LINE jack on the Simon XT and it will work fine.
If you want to have proper line seizure, use a 4 conductor wire to run from where the phone line enters the home BEFORE it goes to any telephones, and run your wire to the RJ31X jack included with the Simon XT and following the wiring shown on page 26 of the Simon XT Installer Manual online.