Accepting proof for one account but not the other

I opened an investment account with FNB, where I sent through my proof of residence a month ago. However, I received an SMS on the number I no longer use and had updated with FNB a long time ago. I also received feedback that the document for proof will not be accepted. Funny enough, when I opened my cheque account and credit account in the past few years a similar document was accepted???? I also explained that I don't rent, and I don't pay utility bills, I stay with my fiance in a house provided by the company he works for, THUS the proof of residence, which is a letter from his company stating I live with him in his company house. I also received with the SMS a notification that the account, with a large some of money, will be frozen unless I provide the neccessary documents, so infact, you will be taking money that does not belong to you and give me no access to it, isn't that theft? I find this whole incident completely unacceptable as I have been a client for years. If FNB cannot accommodate me, then please return my money to my checque account and close the account off, I will take my business elsewhere!