I think whether or not WFH would be taken from all or just one depends partly on the employer - but if it was hard to get them to agree, and it's only been in place a year, then there probably is more risk of everyone losing it.
We can work from home from time to time, though we are expected to come in on days with team meetings and so (some others can be done by confirm call.) We're expected to come in the majority of the time because in the past, we did have a colleague who took the piss. (I don't mind - I live alone, and too much WFH is not good for my mental health. But it's useful to have the flexibility when the car breaks down or not feeling 100, but okay to work, or there are childcare problems - also, we sometimes have to work our of hours and do on-call, so it balances that.)
Take it up with the slack colleague, say that last time it was mentioned, she improved for a week or so, but now she's slipped back, she's just having a laugh and it's really impacting on the rest of you. Say that if she doesn't sort it out, or if she slips back again, you'll be taking it up with management, no more warnings.
I think your management should be aware already, though. We know which of us aren't as productive, whose health let's them have a bit of slack, and we let each other know if we are going to be out for the next hour doing a school run or a doctor's appointment or whatever. But we each door our 40 hours (and in the case where that wasn't true, management was on it.) Also, we have individual targets as well as team ones, so evenue if you slack off on the team work, you can't get others to cover your individual stuff.
I don't always answer the phone or IM instantly - I might be talking to someone else or concentrating hard on something, and I do not need a reputation where I am at everyone's beck and call instantly. Asking them to call back at 3pm when I'll have time for them seems to make people more respectful of my time - and it means I can concentrate on them properly. But I do get back to people, and don't take the piss, so it should be clear I am working.