i find the call drop is more after updating to 8.1.
Sometimes, without dropping of a call it gets mute. person in the other end speaks but not able to hear me.
i have checked whether i have pressed mute key accidentally, but it was not the case.
second thing is can somebody help to clear the RAM and switch app to external storage from internal
Please refer to your mobile operator / carrier regarding the connectivity issues. They should provide a replacement SIM for free, to rule out any potential issues with that, and can advise regarding their coverage.
SIM cards can be defective and wear out over time.
I suggest not to worry about free RAM. Modern OS's move as much as practical into RAM for faster access, and clear it when needed.
Legacy 'system cleaner' (and antivirus) apps cannot work since Android 7.
Make sure to use a genuine, reputable SD memory card at least UHS-1 speed class U3 in the phone, especially if you consider use the SD for apps.
I use the SD card for pictures, videos and documents only.