If you are learning german with the intent to come to Austria, be prepared for a little surprise ...
Some carriers might block calls if the call has no caller id in the sip FROM. I stumbled upon such an issue while forwarding inbound calls from FusionPBX to a cell phone number. The calls were rejected if they reached fusionpbx with anonymous as the caller id.
Reusing passwords is never a good idea and by default the fusionpbx setup would use the same user and password for all domains when provisioning.
If you want to play around with mikrotik or a need a mikrotik CHR, you can install it on XEN and run it without any issues.
Domains transfers normally cause no downtime and no issues but if some registrar want's to play rough it can definitely cause some headache.