You can add up to 5 handsets to this Verizon cordless phone system.

No, they have a whole different phone system and the electrical is a different voltage.

One can purchase 4-Line Cordless Phone online from retailers such as Amazon. Once on the page, type "4-line cordless phone" into the search field at the top of the page and press enter to bring up the phones.

Yes, this is VoIP compatible.

the Vtech DECT 6.0 Expandable Three Handset Cordless Phone System is a good one.