The outside two are the microphone and the center two pins are the ear piece connections. I haven’t used one in years since I normally just replace the entire handset cord when there is a problem. Also, there are two configurations in common use on the 8P8C jack that is commonly referred to as RJ45. Only the USOC wiring is really RJ45 and that is almost never used anymore. When these cables/jacks are used in computer networks, either EIA/TIA-568A or EIA/TIA-568B will be used. When used on a T1 it is actually the RJ-48C configuration that should be used. RJ11, 12, 14 White/Blue pair is on the center two pins, White/Orange on the next two and White/Green on the outside two. The first pair has the T/R reversed. If you are using flat cable, it is automatic if you put the clip on the same side on both ends.