You can use this technology

IVR telephone notification software broadcast voice messages by phone. Ideal for group event reminders, marketing, lead generation, political campaign promotions, school fundraising, church communications, emergency notifications, and much more.

When the event in Google Calendar raises up,call user in IVR

Since all these functions are implemented as a HTTP client communicating directly with Voicent Gateway, they can be run on any machine that has a connection to the host running the gateway. The C# interface source code is included at the end of this section.