Honestly, you're pretty lucky they are contacting you. I'm a single mum with 2 primary aged DC, on with SEN, and a toddler, all home and was trying to juggle full time work around it until I was recently made redundant, so you have my full sympathies - but your kids need support, as do you. The fact that you're not ready to answer the phone just demonstrates this.
My school know that we are focussing on phonics, English, maths and any of the rest is bonus, and that I may be doing work a bit out of the set times and days, but that's ok, and since we've established it, it's been a lot easier. I can do some things on the weekend, or of an evening.
Do you have issues with lack of devices? There may be local charities that can help, or technically you might be able to get a school place. If it is lack of time, then maybe they can help you with some structure, because realistically the high school child should be able to do some by himself and even at primary level they do not need constant 1-1 support, it should be possible for you to do some home learning.
There are a lot of kids doing lots of work right now, you have it hard but that's why you need to talk to the school - they're on your kids' side here, work together to find a way to help them.