You can simply remove the RJ45 jack and put on an RJ11 jack. You just need to identify which cables within the cable you need to connect.
RJ45 Jack= The Ethernet jack that consist of 8 slots for the 4 pairs of wires
RJ11 Jack= The jack commonly used for phone. This connector only has 4 slots for wires
as you can see the amount of wires that each jack can hold differs.
Since you cannot connect your pc directly to a phone jack via the Ethernet port, due to the difference of communication. Your Ethernet port communicates in binary and your phone jack is based off variable or audio transmissions. in order to make this connection you need a supported medium between the 2 different medias, these are commonly found on dsl lines (DSL Modem)
What’s the end result that you are trying to get? It’s usually easier to just get a phone cable.
If your intention is to just replace the 1 end of a Ethernet cable with an RJ 11 you will need to distinguish which pairs of the wires to use. The standard dictates that the 2 orange wires are used to transmit and the 2 green’s are used to receive.
Hope this info was useful if not you can contact me at IThelp1509@yahoo.com if i know more about the project i may be able to offer further insight. Hope this Helped = )