Why doesn't a FaceTime call go through between Ipad mini and ipad air if mini is not connected to 3g?

Question

mobile connection

Hello!
There is an ipad mini with 3g and an ipad air also with 3g. Making a FaceTime call. If both devices are connected to 3g at this moment, then the call goes through and there are no problems, but if the ipad mini is connected to wifi during the call, then the process stops at the moment of connection. It says "connecting", but still not connecting. This applies to both outgoing calls and incoming calls.
There is another ipad mini, and calls between minis pass without problems. But between mini and air here is such a mutual misunderstanding.
So it goes.