Muffled phone calls

[ThunderSpook]

Bit of a strange one that I hope someone can help with.

My friend has a Samsung Android phone on the Vodafone network. If she phones me on my iPhone SE (Three network) it sounds muffled. But if I phone straight back it’s perfectly clear.

Apparently when I phone it shows on the Android phone as being an HD voice call, but when she phones me it doesn’t.

As an experiment I got her to phone my work phone (an old Nokia on Voda) and it said it was an HD call.

Any ideas why it wouldn’t do an HD voice call when phoning my iPhone particularly, but when I phone her it’s fine.

[ThunderSpook]

Thank you for your detailed answer, makes interesting reading.

I take it from that that there is no simple solution then, other than for us to both be on the same network?