Does Spanbox support Rocket.Chat?

Yes, Rocket.Chat is one of the apps Spanbox supports. Add it from the app list, sign in on Rocket.Chat's own page (WhatsApp is a QR scan), and it lands in your unified inbox next to everything else, team channels. On the free plan it just uses one of your 3 app slots.

You can even run more than one Rocket.Chat account at once, each in its own isolated session, so a work and a personal Rocket.Chat sit side by side without logging in and out.