When you accept a call, the phone availability option would turn Orange and be displayed as Busy. After the call is disconnected, this button would return to Green and Available.
However, if you are actively using Freshdesk on Multiple Tabs, or because of browser cache overload, this option would sometimes stay in Orange. So, incoming calls would not ring for you, as you would be displayed Busy in an Ongoing Call.
To overcome this instance, please clear any other tabs in which Freshdesk is open and clear the Cache and Cookies on your Browser. An easier and temporary hack is to click multiple times on the availability button. This would make it turn back to Green and you could continue accepting calls through Freshdesk.