I have tested iNum calls from Blink, SIP client for Windows - Beta Version.
Blink, a state of the art, easy to use SIP client
I used sip2sip free account.
SIP2SIP - free SIP account for Voice, Video, IM and Presence All calls to the Echo test was connected with great voice quality.
Code:
00883510000000091@81.201.82.25
00883510000000091@callcentric.com
+883510000000091@proxy01.sipphone.com
00883510000000091@sip.freecall.com
00883510000000091@sip.voiparound.com
00883510000000091@sip.poivy.com
00883510000000091@voxalot.com
I could call to my PBXes
username@pbxes.org without problem.