What dictates if "Facetime" is offered for a contact on an IPad?

[NeilF]

So, I'm setting up an IPad for a friend, and she simply wants to be able to Facetime her kids who now live over in Australia.

In her contacts she has her two children's telephone numbers which are both IPhones. But, neither offered "Facetime".

I added my son to her contacts, just with his (IPhone) phone number, and that did offer "Facetime".

I then added an email address of one of her kids to their contact details, and bingo, up popped a "Facetime" option.

So what dictates if "Facetime" is offered?

 

[NeilF]

So if I find out what the two kids have set on their devices as regards Settings > Facetime, and ensure that (telephone no./email address) is listed in my friend's contact information for them?