Return:
Returns 'ok' with 'cause' = 'nonexistant' if the address is available.
+ TODO: Returns 'failed' with 'cause' = 'domain' if the domain is not a kamailio domain.
Returns 'failed' with 'cause' = 'exists' if the address is in use.
Returns 'failed' with 'cause' = 'invalid' if SIP address is malformed.
Some TODO's listed above ...
Permissions! (user/permissions?user=...)
+The permissions bit-flag seems to be:
+ NOCALLS 1
+ EMERGENCY 2
+ INTERNAL 4
+ VOIP 8
+ NATIONAL 16
+ SERVICES 32
+ INTERNATIONAL 64
+ OTHERS 128
+
+can_write() checks!
+can_read_full() checks!
Improve robustness of change_pw: fetch old password for rollback/testing.