Do not have the iPhone but have you checked your setting to make sure that it has calls turned on. On Android there is a cog and you can turn on call and audio.
Android also has a media area and you can change the audio on phone or device.
Do not have the 3 pros but as stated not many volume issue that I can remember. Worse case you can contact Service@soundcore.com
If possible connect them to a different device to eliminate the phone. Say connect to another iPhone and same issue. Connect to android and see if they work fine or they work fine on this iPhone but not the other.
As seen , I can use this media area and change the volume of either of I want and that came notication area
This control came from the Android volume control. So it could just be a volume setting just got turned down