When Configuring your 3CX please make sure to put the SIP Username in Custom Field value box in the line From: User Part Without this change you may not be able to make outbound calls.
3CX mobile apps on smart phones may need specific settings in order to receive incoming calls. In your 3CX admin portal, perform the following changes: 1) Go to Extensions 2) Open the affected extension 3) Select "Phone Provisioning" 4) Under "Your Phones", select "3CX App" 5) In section "Network interface for registration and provisioning", select the FDQN. Ensure you click "OK" at the top of the page to save changes.