How do I get my kids to stop using their phones all the time?

Set limitations. You're the parent do not be afraid to set a limit if you feel it's necessary and enforce it.

Replace phones with another kind of entertainment. If they're able to cope without their phones for a few hours, reward them with something fun. It's hard for kids to leave their phones alone so don't be too harsh if they "fall off the wagon". Kids need encouragement and support with breaking habits.

what I would do is take them out to do free and amazing family time activities! Since its winter here in USA, I would go play in the snow with them, make gingerbread houses and other fun things!

You could make them active by engaging them in other activities. You could have a schedule for them when they can have their phone and reassure them you are doing it for the best. You could encourage them to spend their time outside.

Take the phones away and teach them to only use the home phone. Tell them that only the people who need a phone are the ones who deserve them

Written contracts can be very effective, especially with adolescents. This teaches them responsabilities and respect. Here is an example : http://joshshipp.com/teen-cell-phone-contract/

If they are using their phones for browsing the web/ facebook etc then you can try changing the WiFi password in your home each day. The they can "Unlock" the WiFi by doing chores around the house or good behaviour.

play the phone game! everyone puts their phone in the middle of the table and the first person to touch there phone has to do a penalty. My friends and I do it at dinner and whoever touches there phone has to pay the bill!

Try to encourage them to pursue their hobbies, don't just talk to them about it, for starters you can involve yourself with them in drawing, painting, dancing, whatever their hobby might be. This will help them get over their addiction to phone. Sometimes, kids use phone because they feel a lack of confidence in them, they don't feel unique, so they hide behind mobiles, you can take them to orphanages and old age homes, and ask them to help people over there. They will start feeling fuller in their lives and so this addiction will go away.

Establish a time where cell phones are off limits. After a while, they will get use to this time and eventually it will bleed over into other times of the day.