Hello Frank123,
firstly I would like to know which casino we are talking about now, can you share the link, please? There are dozens of casinos with "vulcan" in their name, sadly I didn't find any Volcanic Vegas.
Allow me to say, that the proof of income is an essential document that is necessary for completing the KYC (know your customer) verification, just say the casino needs to prove that the money you deposit truly belonged to you and also that the payment method is associated to you. You see, there is an AML (anti-money-loundering) rule, that serves to protect casinos from participating in fraudulent transactions commonly known as money laundering.
You can learn more in this thread.
Normally, the casino's Terms & Conditions state which documents you are supposed to provide during the KYC procedure. This is a document I find very hard to replace with another option, sadly.
May I know what payment option you used for the deposit, please?
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