By default Apple's Safari browser blocks all auto-playing sounds from any website. Because of this, incoming calls ring sound doesn't work in Safari but there's an easy way to solve this. Also, this issue can sometimes happen in Windows.
How to enable incoming call ring sound in Safari
Go to Safari > Preferences > Websites. Find the Webphone domain name, the most easiest way to do it while a Webphone is opened and set Allow All Auto-Play. After this all incoming calls that come to the Webphone will have a ring sound even when you're browsing other websites.
Find the Webphone and set Allow All Auto-Play
Check website settings in Windows
If you don't hear an incoming call ring sound, check your browser website settings. Usually, a dedicated button for the website settings is placed near an address bar.
Find a sound field in website settings and make sure it's set to Allow.