Are "faoi cheann" and "i gceann" interchangeable? In FGB, they have " Beidh sé anseo faoi cheann trí huaire" but "i gceann tamaill beag."

If I want to say "My daughter will be calling me in a few minutes," for example, would it be "i gcúpla bómaite," "faoi cheann cúpla bómailte," or "i gceann cúpla bómaite"?

Oh these gap in my education!

Redwolf

I'd say faoi cheann/i gceann are interchangeable.

And you can say i gcúpla bómaite as well.
But the meaning is perhaps a little bit different: after (faoi/i gceann cúpla) or within (i gcúpla) a time span.