If you are getting any problem in your HTC Desire 620G dual sim phone then you can try to perform a factory reset (also called a hard reset or master reset).This will erase all your Phone data but solve your problem.
Please Note: Factory reset will remove apps you’ve downloaded and installed, your accounts, as well as system and app data and settings. Be sure to back up any data and files you want to keep before you do a factory reset.
Factory reset may not permanently erase all data from your phone, including personal information.
If problem still exist then you can contact HTC using LIVE CHAT and they may help you resolve your problem online.