Problem with phone calls

Good morning to everyone, I have a huge problem with my iPhone 11 Pro Max. When I try to make a phone call with low signal the person at the other side of the line can’t hear me. It’s not a problem with the microphone because if I use messenger or Viber they hear me very well. Maybe it’s a problem with the signal but I don’t know how can I fix it. It’s unacceptable if I can’t use my telephone. My brother yesterday was in hospital and I couldn’t speak to learn how he’s going…. Thank you in advance!