Your pops doesn't want to add it because

A) his deductible is probably over 500, most likely 1000 bucks. Much more than your phone.

B) If he did add it, and if you broke it, it would be a homeowners claim against his house. You are only allowed so many per company. What happens when he makes a claim on your phone, then next month, someone steals his TV and golf clubs, you get cancelled, then a snowstorm caves in your roof? That phone doesn't seem so important anymore, does it?

C) It's nothing you can "add". You are a household family member. What is yours is his. If you moved away to college taking everything with you.. Had a fire, all your stuff is covered under HIS homeowners policy. Until you're married, out of school, or 25. So technically, it is covered under his policy. It's just not worth enough to worry about.

Listen, I don't know how old you are.. If you're under 18, there's a high chance you can't even take out an insurance policy on your own. Have your dad sign you up for the T-mobile one, or have him call a State Farm agent locally and ask about a P-A-P. Otherwise, buy a foam case and wrap it in bubble wrap, son.. You break it, you buy it.