Unable to hear calls from private number

[Gc25007798]

I am trying to receive calls from a health care service and it's from a private number, it rings but when I answer we cant hear each other. I had the same from emergency nhs caller 24hours ago. I use a s23 model. Any suggestions as why? So frustrating 

[TarynH]

Did you have any helpful answers as this is driving me nuts too.  All from the hospital and most urgent