Yes, that works fine.
First make sure the desired number has been verified through the PC softphone.
In your Nokia handset go to
Tools | Settings | Connection | SIP settings and edit the
Public user name to match the following format:
[number in int'l format]@sip.[brand].com
e.g.:
+44123456789@sip.poivy.com
or
0044123456789@sip.poivy.com