A number of magicJack plus users are finding it difficult to hear others on their phone. Likewise, others have trouble hearing them.
Here’s a fix that should work for most users.
Update 2013-10-08: Originally, I thought this volume fix worked. After hearing back from Kher and Bill (commenters below), I performed multiple tests on both the magicJack 2012 and 2014, I find that settings revert back to what seem to be fixed settings for the telephone volume controls when plugged into the router. There is no way to adjust volume settings while the device is connected directly to the router. This is a real problem that magicJack needs to address. There needs to be a way to directly change volume settings while connected to the router. The only way this is likely to happen is if the device hosts a device web interface. Hopefully it will be addressed in some future version of the firmware.
With the magicJack plus inserted into the USB port of the computer. The magicJack interface should pop up if you’ve already installed it. If you have not, go ahead and let the installer finish installing. You should then see the computer magicJack interface.
From the menu, pull down to “Volume/Headset Control”. Make sure you have “Telephone” selected at the top of the dialog box. Change the volume for “Speaker” and “Microphone” at the bottom of the dialog box. Close the dialog box. Close the mJ interface. Unplug your magicJack plus from your computer and reinstall it on the router as normal.
Your volume should now be at reasonable levels. If this doesn’t work, you may need to return your device to magicJack.