You need to have bigger income than the plan you need to avail.
Are you already an existing subscriber/planholder or just applying?
Because if you are then why not try to call first and ask if your credit limit could be increased based on the documents you have submitted before? It is only then that you could find out if you need to re-establish your credentials or submit other documents to justify the increase.
If you are a new subscriber then you would have to present documents which would justify your requested credit limit.
Frankly, I do not understand how Globe establishes or assigns the credit limit to its subscribers. I have never requested for any increase and some of my accounts have actually exceeded the credit limits given by Globe. To add some mystery to it, my low cost plans have a high credit limit while my highest plans have a low credit limit (lower than my lowest plans) which I often exceed yet are not cut or given warnings.