SMS verification was carried out when the account was created, but the game such as Iron Rider required SMS verification again, again verification, that is, the phone number is blocked, and other phone numbers can not be switched.
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Once you have chosen your country you do not have to input that county code again. When you input your phone number do not use the 0 (Zero) see example below 0123456789 instead input 123456789. Some options to try if it is not working, clear your device cache and try using an incognito browser.