SIM800l “Call Ready SMS Read” not consistent.

Using SIM800l I am sending SMS and calling URL. Functionality is working fine.

Sometimes I get “Call Ready/SMS Ready” and sometimes I don’t get. Functionality works fine all the time.
Problem is, while sending AT commands, suddenly I get “Call Ready or SMS Ready”. If I wait for it in my state machine then sometimes it never comes.

Can anyone please explain how to make it consistent?

I am giving 4v to the module with DC – DC buck converter(LM2596).
I Connected GMS Antenna.