|It is not recommended to combine LAN and telephone wiring into one cable. Some of the reasons have been given earlier in this thread.|
Previously some people used a method to do it but have abandoned the practice because of growing sophistication of network standards making with RJ45 configuration utilizing previously unused pin-connections. It is not cost-effective now since the cost of wires and components has come down. It can become incompatible as the newer networking standards come online.
The information and caveats below may help you to make your decision. You may realize that no longer the effort is worth the return.
An issue unmentioned in the threads earlier is about incoming phone lines. In the older methods of providing telephone access, still in place in many communities, the incoming phone lines are potential entry points of huge voltage surges from lightening or other sources that can fry up the adjacent network signals with unpredictable results to your network. Some phone equipment has surge suppression built into it, but your network may be vulnerable. Not a daily event, still it is an important consideration.
Although I do not recommend it, but if you want to tinker at your own risk, use the blue pair on CAT5 wired as 4,5, for red and green of RJ11 and use 1,2,3&6 (orange and green pairs) for ethernet. In RJ45 the blue pair corressponds to the 2 center wires of RJ11 (red,green) automatically.
In RJ45 (T568B standard), pins 4,5 (blue pair)and 7,8 (brown pair) are undesignated. Therefore they could be used for purposes such as phone traffic. In a single line service, RJ11 and RJ45 corresspondance is via the 2 center wires is inherent. For two incoming phone lines there is no RJ11 and RJ45. Non-standard connection of 2nd line via pins 7,8 is theoretically possible but not a good idea.
If you just want to use CAT5 for telephone wiring then consider the following:
CAT5 connectors (RJ45) have 8 contacts (4 wire pairs); standard telephone jacks (RJ11) have 4 contacts (2 wire pairs).
If you have 1 phone line, use the center pair (Blue/Blue-white) in the CAT5 cable, and cut-off/leave all others unconnected. Connect the CAT5 pair to the R & G (red and green)-of the standard phone jack.
With 2 or more lines, use one of the twisted pairs in the CAT5 cable for each line, strip the sheath from the other end, and connect each pair to a separate RJ11 jack for each line. Then distribute the lines accordingly.
RJ11 can accomodate two lines. Some multi-line phones and adapters can separate/use both lines. Some older phone modems/equipment may not operate correctly if 2 live lines are connected to the input. If needed, this can be prevented by using a line cord to the equipment with only 2 wires connected through to each end.
Another helpful piece of information is the standardized color order of RJ11 and RJ45. This can be easily found through an internet search. Basically, the order on RJ11 is BRGY (blue, red, green, yellow) on one end of the cable and reversed on the other end so that B always ends up with B on the opposite end, R with R, etc. The socket side corressponds accordingly. RJ45 is wired straight through or crossed-over. Consistency in keeping the standard should be maintained to avoid technical problems and future confusion.
Hope all this information helps