You don't answwr, I hope, other than congratulating them?

That sucks. If people are still messaging you for help, that's a sign of how good you were at your job. :(

setup a company and send a bill for consultation fees. put in copy all the messages your replies and time spent look at the competition and show them how valuable you are

Time for some revenge: tell them you can help them but that you'll need to be pay as a consultant. Reddit is full of stories like this if you need. If the new staff still asks for your help and your old boss doesn't want to pay you, then just tell him you no longer work for him so you can't help them.

If you keep helping them for free they're going to keep calling you

I certainly hope you have been able to find another job since then! It sort of sounds like after you trained the shift, they fired (hopefully actually laid off, making you able to receive unemployment benefits) the most expensive person - you. And they kept all the cheaper new hires. Real A-hole move on management’s part! While you might want to help your former trainees, when you answer their questions it just makes it all the less likely that you would be rehired. Why pay someone when they are doing the support job for free? Cut it off now - No more free help. This isn’t helping you at all, and you might be a crutch for the current workers who ask you instead of figuring it out themselves or going to their boss like they should.

So don't answer the texts

Free is the best price.

Don't respond, make the bosses know that they can't do it without you.