Pho gets into sleep mode even while in use

My iPhone screen turns black or get dim randomly at anytime even while in use such as I am writing some message on wats app and it turns into sleeping mode suddenly and then come back to normal stage with in 2 3 seconds. While I talk to someone it gets into sleeping mode again and if I want to keep it on loudspeakers I’ll have to keep the person on hold to try n click every available button to get it into normal mode . So that I can put the call on loudspeaker. This behavior doesn’t Persist all the time . It is very random. Is that a phone problem or am I missing some sort of settings?