I have a problem that sound quality decreases, and in addition, if the conference, the speakers do not reach agreement
Please tell us which is the exact version of Zoiper that you are using?
What is your operating system and what audio device are you using? Can you share more details about the issue?
I use Zoiper Biz 3.9 for Windows under Windows 7 with a Jabra headset brand
Please make sure that you are using TCP transport type with your SIP account.
The audio quality depends on the network, the network load and quality and the audio codecs that are used. You might want to report the issue to your provider and ask them what are the recommended codecs to use with your account.