Things you might want to check :
Phone is using the latest iOS that is compatible
Turning phone off and back on ... any difference ?
Quitting and re-opening FaceTime with Apple ID Log-in ... any difference ?
the settings for "mute" on / off are working ok in FaceTime
your input sound volume
Eg: Headphone notifications on your iPhone, iPod touch, or Apple Watch
https://support.apple.com/en-in/HT211903
Volume on iPhone
https://support.apple.com/en-auhttps://support.apple.com/en-au/guide/ip
iPhone sounds and vibrations
Go to Settings > Sounds & Haptics (on supported models) or Sounds (on other iPhone models).
https://support.apple.com/en-au/guide/iphone/iph07c867f28/15.0/ios/15.0
Do Not disturb settings:are ok
Airplane mode:
https://support.apple.com/en-us/HT204234
Screen time settings.
Also, if needed
how you set up FaceTime, (there may be a setting you need to change ?):
Set up FaceTime on iPhone:
https://support.apple.com/en-au/guide/iphone/iphc4774d8d8/15.0/ios/15.0
Make and receive calls on FaceTime:
https://support.apple.com/en-au/guide/iphone/iph7801d5771/15.0/ios/15.0
Wondering if you use wi-fi or cellular for FaceTime ? :
( If cellular check with your provider ?)
View or change cellular data settings on iPhone
https://support.apple.com/en-au/guide/iphone/iph3dd5f213/15.0/ios/15.0
Any third-party (non-Apple) apps that may be interfering.
All the best :-)