True, you can't call them "backup" to their face. But you will trickle through traffic for weeks whilst you sort out teething troubles anyway, so there is not going to be volumes after a few weeks.

Then eventually you can make a decision which works best. The new provider (the backup) or the old provider. Then turn the volume to the best one.

Though I would never go 100% on any of them. Maybe keep some geoip through the backup. E.g. South America with one provider. France with another. Or a 5% load balanced volume.

It is good to keep them all up to date and active, as from experience they all frequently f-up and you have to feature toggle a provider off for a day or week now and then, or at least throttle them down.