It depends. I don't think there is code for low voltage wiring (someone please correct me if there is code) junctioned out of reach/in a wall. Main reasoning here is that I have seen many many phone and security lines installed and have seen these guys splice together lines everywhere. I don't think their companies would train them to do this if it were against code.
However.......... I have also got permits in a lot of cities. No inspector I have dealt with will let me bury a utility access point in a wall. And given that there may be an older owner there next that may use the phone lines in good conscience I would put in a $2 rubber box and a plate on the wall - they do sell paintable blanks for stuff like this.
(Alternative is just note the EXACT location - picture with tape on the spot. Know that if you need to access it you will need to cut open the wall there. Know that you will disclose this if selling the home.)