Making custom length RJ12 cables? Use these RJ12 6P6C Plug to crimp the ends of your RJ21 patch cables to make sure that your cables are just right for the job. This RJ12 6P6C Plug 5- pack can make up to 25 cables. Monoprice Cable Flat Stranded RJ12 6P6C Plug (50pk) . Monoprice 50-pcs RJ12 6P6C Plug for Flat Stranded Cable.