Nexus 5 signal drops immediately when making call

My Nexus 5 running on Marshmallow and it has Last ota security update provided by Google at the time of end of support. I’ve a perfectly working SIM card. Using this sim card to make calls from nexus 5, but before the call gets placed the mobile signal drops to zero. When the call is disconnected the signal resumes back to normal. I tried soft reset, hard reset and airplane mode on/off. But nothing works. Any help would be appreciated.