You may encounter this issue if you sign into Cisco Jabber for Android on more than one device. Manually sign into Phone services to restore connectivity.

To do this, open the navigation drawer and tap Phone Services. Verify your credentials and tap Save.       

Contact your administrator if you’re using a secure phone service and you’ve reinstalled Cisco Jabber for Android. The security certificate previously saved to the SD card is reset during reinstallation and will need to be reconfigured by your administrator.