What I've seen in the past, is their server time clocks get out of sync. So on their side, the registration expires early, like you said, by as much as a minute or more. Then your device re-registers, and all is well for a few minutes, until early expiration happens again.

The Grandstream ATAs sold by Voipo have a setting that allows them to re-register early. That helps avoid that little gap of downtime. But many BYOD devices do not have that setting available. So they rely on accurate clocks.

I'd suggest opening a ticket, to have support check their server clocks, and sync them if necessary.

Also check time on your own devices, and sync them if necessary.