An odd problem. I have a one plus with dual sim (1 Bt, 1non Bt) and my wife has iPhone 8 with bt sim. When I call her using the bt sim she just gets noise whether using the phone or speaker. No problem if I call her using my other sim or WhatsApp. She has no problems receiving any other calls and can call my bt sim without problems. We have tried restarting both phones. I am guessing it is a carrier issue rather than hardware. Any ideas? Thanks in advance.