I'm as poor as it's possible to be in this country - lone parent to one teen, zero child maintenance, low paid care worker, and renting.
Yet, I accept that it is my responsibility to feed and clothe my child.
If OP genuinely can't afford to, then she needs to look at ways of increasing her income. That could be Universal Credit top up or extra hours. Child Benefit is £96 p/m, and if you don't have enough income to feed and clothe your children, then it's likely you'll be able to claim that. It is meant for the child, not for his share of the mortgage and council tax and TV licence.
DS gets free school meals (residual) but it's not really enough to keep a growing teen going all day.
So, I top it up by £10 p/w.
I also give him £10 p/w 'expenses' for other drinks, sweets and snacks, which he 'earns' by doing the rubbish and recycling.
Teenagers don't always want to take a bottle of weak squash and a bag of Lidl crisps to the park/football pitch/basketball court. Here, they all go to the village shop, together and they do share/treat each other.
We both have refurbished, older versions Google Pixel phones, and I pay £10 p/m for his phone, because it's a necessity - I have to leave him EOW, and school holidays, while I work. I can't imagine 'charging' him for his share of a family/household expense, so I'd no more calculate his share of the SIM package, than have him pay his share of the broadband or the petrol for days out, from his pocket money.
I put £25 p/m in his bank account. This covers for his Playstation subscription and his football 'fanclub' membership that allows him to listen to matches, as we don't have Sky Sports. He uses the rest to buy, eg, new footballs, as he gets through a lot of them, and we go halves on things like football boots, football strip, more expensive trainers, if he wants them and reasonably sensible things like a bag or the nutri-bullet he wanted.
I pay for cinema tickets, but if he wants more than a £1 worth of tuppence and a few games of air hockey at the arcade, he pays. And if we eat out and he wants a second Subway or an extra drink (he always does), then he pays for that!
His paternal grandparents transfer £50 a year at Xmas, and again on his birthday, from all of them, including his Dad. That's the only money he gets, outside the money I give him, so he can't rely on birthdays and Xmas to boost his savings.
So, 14 y old gets £125, plus £55 for music lessons p/m, his basic clothes, and halves on extra shoes and clothes, as and when.
It sounds - and feels - like a lot, but I can't see how any less would be fair, or reasonable. It helps that he is a good kid, never fully empties his account, really doesn't ask for much, and it makes me happy to treat him.
In answer to OPs question, I would have gone halves on the phone - and a protective case.