Well their FAQ is quite clear, I doubt a self-install changes anything, I believe all they are saying is there is one phone jack on the modem for each line.
»optimum.custhelp.com/app/answers ··· a_id/263
You can connect as many phones as you'd like to Optimum Voice®.
1. You can connect multiple phones to Optimum Voice utilizing existing phone jacks in your home. Cablevision will connect Optimum Voice to your existing working phone jacks if you ask for our free Home Wiring Solution. There is no additional charge when its requested with your initial order for Optimum Voice service. Otherwise, a professional installation fee of $39.95 will apply.
2. You may also use multiple phones with Optimum Voice by connecting a multi-handset cordless phone base station to Line 1 of the cable modem. Additional satellite handsets associated with the base station simply need to be plugged into power outlets. Click here for a diagram.
Warning, If you post nonsense and use misinformation and are here to argue based on those methods, you will be put on ignore.