I use dtac, get a cheap tourist one at the airport (they will register it for you)then download the app and select what package you want. For international calls they charge 7bht a minute to landlines
Salary doesn’t matter, if you’ve got the 500k equivalent in your uk bank account can one of your employers write you a short letter stating that some of your work can be done remotely? Thats what I did through London embassy.