Sorry, I was answering the OPs question, of not passing on the initiate call function. I would not expect USSD codes to necessarily work, especially the ones that are handled by the phone app itself. CALL sends the digits to the dialer, not the app. Codes that are passed on to the phone company, such as *67 to block the caller id, should work though. And I verified it does on my phone.
Sorry, I was answering the OPs question, of not passing on the initiate call function. I would not expect USSD codes to necessarily work, especially the ones that are handled by the phone app itself. CALL sends the digits to the dialer, not the app. Codes that are passed on to the phone company, such as *67 to block the caller id, should work though. And I verified it does on my phone.