Call Rejected

I’ve started to receive a message ‘call rejected’ when I phone a particular individuals number. What seems strange to me is that up until today that didn’t seem to be a problem. She can phone me but I can’t return the call.

One thought occurred to me my friend has a habit of not answering calls and I might call several times and yet I got through to her first go

Im calling from a iPhone XR while she, I think is still on is using an iPhone 6

Any thoughts or suggestions