The Home Signal unit functions as a filter, only registered phone numbers are accepted. The only factor that three require is the 'phones number, They already have that, since the SIM when fitted in the Gemini works. Putting the SIM in the Cosmo also works when in an area where there is a signal. VoWiFi also works. Setting up the connection also works, up to the point when the setup is complete. It would appear that the Cosmo does not like the Home Signal, or the Home signal does not like the Cosmo. Since the Home Signal likes the Gemini, it rather looks like the Cosmo has a problem. If the somewhat uninformative signal light on the Home Signal, is to be believed, there is traffic between Cosmo and the Home Signal unit while the setup is in progress
I've set up the VoWiFi on the Cosmo, and it works fine. The only snag, here, is that the Cosmo concludes that there is no phone connection and puts up a message that only emergency calls are permitted, despite the VoWiFi symbol being displayed next the the (empty) triangle of the phone signal strength, and phone calls work coming in or going out.
I'm quite happy with the VoWiFi, and since the house is stuffed with WiFi relays, the effective range of phone connection is better than that of the Home Signal unit which lamentably, is a bit short on power. At one time I had one of Vodafone's femto cell units, and that had a range of some 30 metres. Three's contribution struggles to manage 10 metres, probably since the house is better than 200 years old, with stone walls two feet thick.
Rod