We rely on both, and I think we've got the mix about right. Text for relatively simple things, when numbers and other identifiers need to be transmitted accurately, voice / video for complex discussions that require screen sharing and / or when there are multiple opinions that might need to be reconciled.
I guess I assumed you were deploying overly complicated glorified CRUD apps like (most of) the rest of us.
So what do you use for comms during high impact incidents? Are you all in the same room?