Default facing problem of network congestion "ONLY" on HTC one

I am a HTC One m7 user...after a year of use i am starting to face a irritating problem where..Anyone calls me it goes blank afterwards it rings after about 30-40 seconds..then i thought it would be a sim problem so i inserted the same sim in old samsung non android smartphone where the incoming calls are received perfectly without any problem..As soon as i insert the sim in HTC one the problems starts..even i have replaced my HTC one..still having the same problem..is there any chance that i my google account settings are somewhat bad.?please need urgent help!
NOTE:1.facing problem ONLY on HTC one
2.carrier Airtel India
3.facing this problem two to three times a day.
THANK YOU!