Not sure if someone mentioned this already, but turning off the WiFi won't work because your kids surely need it to get their homework done. My 13-year-old views all her assignments on the school's online assignments system every day and has to go online to complete and submit many of her assignments in nearly every class. In geometry she must watch online videos of the teacher's lectures every day. So just turning off WiFi isn't an answer, unless your kids are in a pretty archaic school where nothing is done online.
I agree with your taking possession of the phones yourself for certain periods, whether via a phone basket (love that idea) or something else. Be sure to do it with advance notice the first time; have a family meeting (include your husband or significant other and be sure he backs you 100 percent and will do the phone-taking duty if you are not around) and set out the rules.
At the same meeting, have a chore chart ready for all the family -- not just the kids but you and your SO as well, so the kids see that all of you have roles to play. Connect the use of their personal phones to their performance of chores if you want, or don't -- that's up to you -- but ensure that you are consistent, whatever you do. If they fuss that they must have a "payment" for chores, explain that there are chores that must be done to keep a household functioning, and the reward for doing those is a functioning household.
Also, if your kids' computers are in their rooms, and you think they are going online to sites for reasons other than schoolwork during schoolwork times, then move the computers to a very public area like the kitchen or living room. The kids will fuss and fume but you have to stick to this; consider whether they can earn back the right to have computers elsewhere IF they prove they can get work done and not drag out homework time by going onto non-schoolwork web sites.
The other way to cut phone usage is to make their phones into PHONES and not Internet devices. If the kids are really rude or resistant about limiting their phone use, I would then turn their smartphones into mere phones and halt their abilities to surf or text or get on Instagram or other things, assuming that's what they're mostly doing. Again, they might be able to earn that back. I wouldn't go this route, though, unless they are really unpleasantly resistant. Don't threaten it unless you really will do it to their phones. But yeah, I for one would do it if warranted.
Our family uses phones to make calls, and that's it. No surfing (and no, we don't have smartphones). Our middle schooler does not have a phone at all because we consider phones to be for needed communication, not for surfing or taking photos or checking in with friends -- she uses plain old e-mail on a computer for that and doesn't need a phone to reach us since she's either at school or at places like activities where we're nearby and there are phones she can use.