Hi vaidyanathan240,
It sounds like your speaker volume is low on calls. We're glad you've joined us in Apple Support Communities. Let's see if this helps.
Follow the steps in If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker. This link has the most frequent solutions to troubleshooting when you're hearing low volume or sound on your iPhone.
If the issue remains after following those steps, let us know what steps you had a chance to try from above and what happened when you tried step two from the link above. Also let us know what happens when you try to play audio or video from a different app.
We hope that helps with your question and take care!