If you stop paying the iPhone bill. Will they take away the phone?

Okay, so I got an iPhone for my birthday from my grandma. She has been paying the bill for me, but now since I'm getting my hair dyed all black. She threatened me to stop paying for the bill. I really don't mind, but I want to keep the phone for text apps and social media.
It may seem like a stupid question, but I never heard anything about it. Anyway thank you if you have helped me in any way.

I'm not sure if you're talking about phone bill or the bill paying off the phone. Either way yes you can keep the phone.

You really think that they'd pay a guy to come to your house to take your phone away.

All it means is you won't be able to text, call or use 3G/4G internet. You will still be able to use Wi-Fi however.

Depends on how vindictive granny is. If she is crazy and reports it stolen, and who the thief happens to be, she could get the service turned off and you arrested. But only you what your family dynamics are, and who can bluff and who is batcrap crazy.

Never play pull my finger with an explosives expert, that loves practical jokes where other people get hurt.

If its on contract it will be blacklisted but you can still use Wi-Fi.