Ringcentral for Salesforce Professional Edition API not enabled

I have a Salesforce Professional edition in which API is not enabled. I have already integrated ringcentral with Salesforce through an app Ringcentral for salesforce which is available on Salesfore AppExchange platform. I have setup the calling with RingCentral For Desktop. I am not getting incoming calls in salesforce and only getting calls in RingCentral Desktop app. Please let me know if API enabling is mandatory in Salesforce to receive the calls or to make calls from Salesforce. Please reply as soon as possible.