You can't just add a phone line option from your ISP? It's all VOIP these days and sometimes it's actually cheaper to get it bundled (if you have a cable modem)

You can't just add a phone line option from your ISP? It's all VOIP these days and sometimes it's actually cheaper to get it bundled (if you have a cable modem) than without. Fiber and DSL usually offer it too. It'll be a bit more than a DIY SIP phone, but a hell lot less hassle. Also, it's probably easier to just use the smartphons' parental controls.