If the customer is disconnected from email and the internet, contact via SMS text message is a cost effective and efficient way to go. Please note that this is a premium feature.
You can turn on requesting a mobile phone number in the support form. You are asking for the information with the express goal to provide support ( or a replacement voucher/user) via SMS text message. This can be switched on in your account control screen in the control panel. You can use the mobile phone number you received through the support to either just contact the customer and advise of a solution to their issue, or even to supply them with a replacement username and password, whatever is appropriate.