My college student doesn't have a smartphone, and she isn't whining about it. Literally, she's never mentioned it to me, so it must not be an issue at all. She was assigned exactly one group project during her freshman year of college. For contacting group members, any old dumb phone that'll call /text will do. For research, sharing GoogleDrive docs, etc., a laptop is superior to a smart phone anyway.
If the girls want them, I'd suggest saying, "I am willing to continue paying X amount/month for your phones. If you want more expensive phones, I will continue paying X amount/month and you may figure out how to pay the extra cost." And discuss contracts so that they know they cannot choose smartphones today and switch back to dumb phones tomorrow. This method isn't exactly untested: My mother used it on me. I use it on my kids today. No, not necessarily with phones, but with plenty of other items. For example, today we went shopping for jeans. I am willing to pay $20, maybe $25, for jeans. If they want $45 jeans, they can have 'em, but they must pay the extra. Over the years, my girls have frequently decided that an item was "worth the cost", but they always stop and think about it.
We will cover all their basics for college (and that does include a basic cell phone -- they don't even offer phone service in the dorms anymore), but they're responsible for spending money and upgrades of all types.