WiFi calling not working over VM (on iPhone)

Anyone else on VM seeing this?

I have a work iPhone SE which is on Vodafone, and normally make use of WiFi calling as the signal on Vodafone is poor in most of my home. The last two days I have had to go and stand by the window for colleagues calling me to hear. So I did a test with my personal EE phone, set the phone to airplane, turned on WiFi and no WiFi calling.

If I use my 4G pocket router and connect both phones to this WiFi signal, the WiFi calling works immediately.

Anyone else??

I narrowed the problem down to my third party router (ASUS), and it turned out you must not turn off "IPSec passthrough" on your own router.

I had turned off all the NAT ALG's on some security advice, and hadn't noticed !

Oops !