WiFi Calling is not reliably available

WiFi Calling is not reliably available on my Android phone. Sometimes it disappears and requires turning airplane mode on and off to re-appear. Rarely a phone restart may be required. It is also unreliable when changing between, for example, home and work WiFi. The networks are different, both good quality, but frequently WiFi calling will work in only one of them. Any ideas what might be causing this? The issue appears on two Xiaomi phones I have, same carrier. Phones are Redmi 9C NFC and Redmi Note 8 Pro.