<user_host idx="INDEX" perm="PERMISSIONFLAG">VALIDVALUE</user_host>
Specify the IP or DNS address of the registrar/proxy where you want to register this account. After a successful registration, the registrar knows how to reach this speciﬁc identity and can route requests (e.g., incoming calls) from other registered parties to this phone. The setting must match with registrar_whitelist.
e.g. for <sip:firstname.lastname@example.org> the user_host would be <company.de>.