How can I make VOIP Call from Website to Mobile Device?

I have a requirement where I need to make a VOIP call from my Website to a Mobile Device. I tried using Twilio. But it seems it doesn't work in India. And the Twilio support is not giving appropriate response. Can anyone suggest me a possible solution for this. I will give you my more in-depth requirement. There are two end users here. One is the Principal of a school, and the other one is a Teacher. Principal is using a website, and Teacher is using a Mobile device. We need to give a platform to the Principal to make a call to the Teacher, through its browser. We are using PHP/Laravel as back end.