Due to the fact, that Android-5.1 requires a lot of more memory than older Android versions we recommend to insert an external SD-card, or plug in an USB-stick with minimum 2GB free memory space, at least during the upgrade phase!
We strongly advise to do a factory reset after the upgrade to Lollipop, to be sure that all new Lollipop features and settings are correct. A stable SIP device is required in a business customer environment. Via save - restore the configuration or provisioning, the SIP accounts settings can easily be restored. For Android settings / Apps, see the Android Backup and Restore procedure.