What needs to be changed in the settings in order for the second call to be silent?
Which Zoiper product are you using ?
Navigate to Zoiper -> Settings -> Audio and disable "Ring when talking".
You can find the option by scrolling down a bit, but it is available Zoiper PRO only.
In the same version, there was no previous problem.
How can I solve the problem in a free version?
Huge gratitude for the help!
I can only suggest that you contact your service provider and ask them to limit the call waiting functionality for your account.