The apps
Zynkrow for Android and iPhone carries Chat, calls, Tasks, Notes, Docs, Sheets and Mail, with more arriving. The web apps work on the phone's browser too.
Signing in
Sign in with the same Zynkrow ID. Your organisation and every app you have appears; a person in two organisations picks the workspace from the drawer.
Notifications
Allow notifications when asked, and open the app once after installing so the phone registers. On Android, keep battery optimisation off for Zynkrow if calls should ring with the phone locked.