The following USSD code once dialed and called as a phone number, will give you access to a hidden testing menu for Android devices.
*#*#4636#*#*
Unstructured Supplementary Service Data (USSD), sometimes referred to as “Quick Codes” or “Feature codes,” is a communications protocol used by GSM cellular telephones to communicate with the mobile network operator’s computers. USSD can be used for WAP browsing, prepaid callback service, mobile-money services, location-based content services, menu-based information services, and as part of configuring the phone on the network.
From: https://en.wikipedia.org/wiki/Unstructured_Supplementary_Service_Data
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