check microphone and reciever in samsung members or dial*#0*#.if they are working properly then try changing sim cards. if still the problem continues wipe cache partition(search in google how to do it).
no command aye gi thori der baad black screen aye gi wahan wipe cache partition likha hoga vol up down se upar neechy kr k power button se ok krna hai bas 2,3 sec mein clear ho jaye gi phir reboot kr lena hai
Wahi jo Saadi77 keh rhay hain smart switch wala wo try kro. cache partitiion bhi clear kr k dekho ho sakta hai phone ka apna masla ho.phone off kr k power,bixby aur volume up ikatha dbana hai.