When using Vivox on iOS with Bluetooth devices, before you can use audio, you must correctly initialize the AVAudioSession category and options.
For information on how to perform this implementation process, see one of the following KBs:
How to: Configure iOS for audio and Bluetooth use (Vivox 5.15.4 and later).
How to: Configure iOS for audio and Bluetooth use (Vivox SDK versions previous to 5.15.4)