The value of username comes from the username part of To URI. What probably confuses you that there is msn1-0003f5 in display name, this is just an informative text used by user agents to display the name of the person in human human readable way. It is optional.
hm, strange, so i send "to: 12345@mysip.de" with Authorization: DIGEST username="msn1-12345678" and then i am register with "12345" ?
serctl ul show 12345
200 OK sip:12345@127.0.0.1;q=0.00;expires=3226
Should it not be a registration with username "msn1-12345678" ?
greets, Markus