I think Credit Limit is not based on your salary. With my 5 lines, most of them just have a little over my Plan.
For example, Plan 999, Credit Limit is 1000 BUT you can always request for increase of credit limit which then they will base on your plan payment history.
Credit Limit is based on your salary or capacity to pay, your current plan and your account relationship / history and some other things.
But it is different from the Credit Limit in Credit Cards.
As to your question, if you are applying for plan 999 and get approved based on your salary, your credit limit will be then be based on your plan, they can give you 1500 or higher credit limit.