Greetings mohandas77,
If the behavior is occurring in a specific location, or if it only happens at certain times, then it would be best to speak with your wireless carrier to check for network issues.
To help troubleshoot, ensure your iPhone is updated, and check for carrier updates:
- To check for an update, tap Settings > General > Software Update.
- To check for a carrier update, tap Settings > General > About (a pop-up will appear if there's a carrier update available)
If the issue persists, reach out to Apple Support directly. They'll be able to check if there's a hardware issue and help isolate the cellular quality behavior.
Contact Apple for support and service - Apple Support
Kind regards.