Duo is an indispensable Bluetooth-based helper for your iPhone, while Duo plus is an assisting app designed for using Duo on iOS devices. Once paired with Duo via a specialized App, your iPhone will become Dual SIMs & Dual Standby, which allows to make phone calls and send SMS with any SIM card inserted in Duo. For the iOS devices without communication functions such as iPad and iPod Touch, Duo can enable them become a real phone. Phone Call & SMS When Duo is connected with your iPhone, contacts list will be synced. You only need to make phone calls and text messages on App as what you usually do on your phones. Duo App cannot edit contacts. If you want to edit contact information, please go to Contacts of your phone to make modifications. Regarding DuoPlus’s background audio play function:

The matched BLE gadget contains Anti-lost Alarm function.
It has 2 modes: Mutual Find and Lost Alarm.

A. Mutual Find Mode
After BLE gadget connected with App, in App’s Settings---More, you can enable “Find iPhone” feature, and select unique Ringtone for alarm from in-built ringtone list.
When user needs to find his misplaced iPhone (iPhone and BLE gadget stay connected), he can long press the camera button on BLE gadget for 4 seconds to make iPhone play selected Ringtone alarm, this audio sound will help to locate and find it. 
During Ringtone alarm playing period, if iPhone found, he can repress the camera button for about 4 seconds to mute and stop Ringtone playing.
Either App is running background or foreground, long press the camera button will make iPhone play this selected Ringtone alarm.

B. Lost Alarm Mode
When iPhone and BLE gadget lost connection (poor Bluetooth connection, App quit abnormally, long distance between each other), this selected alarm Ringtone will also be triggered to be played on iPhone to alert user, no matter App is running background or foreground.

 Regarding DuoPlus’s GPS function:
If iPhone and BLE gadget lost connection, App will use iPhone’s GPS to locate and record the current location of iPhone, in App’s Settings---Lost Records, this last connection location will be marked as Pinpoint on Map to help user backtracking it.


Related functions demonstration video as below:
https://youtu.be/HZbE4vaxKQw Supporting devices It supports Apple devices with iOS7.1 or above. (iPhone5, iPhone5c, iPhone5S, iPhone6, iPhone6 plus, iPhone6s, iPhone6s plus; iPad2 & above; iPad mini all series; iPod Touch5 & above) Notice: Continued use of GPS running in the background can dramatically decrease battery life.