Phone barely rings when receiving a call

So this is a question for my mom’s iPhone 14 Pro Plus. When she is on the phone with anyone and someone else calls her, the phone will do a half ring and then go straight to voicemail. She said sometimes when she is on the phone with someone and someone else calls her it will show up that someone is trying to call in like it should but it’s not often that it happens. When she is off the phone, phone calls work as they should. She said that the call does show up in her recents. What are some steps that I can help her with to get this problem solved?