... since I have a dedicated smartphone.
By smartphone do you mean a VOIP phone or a cellular phone or something like that?
One of the best benefits of a traditional landline phone (not a cordless system) is that power to the phone is supplied by the phone company. The power continues during a power outage. A cordless phone system will go dead during a power outage unless the base unit has battery backup.
You are aware that burglars can cut power to your home rendering the magic jack useless?
If the magic jack phone pc, dsl/cable modem and router are all protected by a battery UPS, then phone service should continue during a power outage.
BTW, I have magic jack, love it and use it extensively for my home based business. I would never use it for a critical application like a security system