Life-long "no" struggler here... screening your calls is a good thing.

If you say 'no,' do they call someone else? Are they calling because they don't have enough coverage? If that's the case, I would continue to say 'no' without any worries. They probably aren't going to fire you because they are already short staffed. They need to solve their problem by hiring more people, not antagonizing their existing workers.

One thing that helped me get better at *not* saying yes was to say 'Let me check' or 'I'm not sure.' Then later on I'd get back to them with 'sorry, can't do it.' It's even nicer if you can email or text your no I also try not to give reasons because people try to talk you out of them.

I highly recommend the book 'When I say no, I feel guilty.'