I increase the sound at 100% and it's not really loud to ear the other person.
But the other person can ear me clear and loud.
Any setup to look ? It's on the Iphone 5C with ios 9.3
Try to use different combinations in the Audio section from Zoiper's Settings menu. In most cases disabling of the Auto-gain control might help, but it is best to test for yourself and to see which combination is most suitable for your.
On my iphone, turning off noise suppression worked