This page might be of use to you.
http://www.voipuser.org/review_29.html................................................................................
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http://www.wirelessforums.org/uk-telecom-voip/sipdiscount-configuration-zoom-x5v...The following works for a SIPdiscount account on the zoom X5v (other
configurations will probably work too):
User ID: Username
Authorization ID: Username
Password: password
Display Name: Username (don't think this is too important)
Domain/Realm: sip.voipbuster.com
SIP Port: 5060
SIP Proxy address:
SIP Proxy Port: 0
Outbound Proxy Address:
Outbound Proxy port: 0
The rest was unchanged from a previous voipbuster setup....
Note: SIP Proxy address and Outbound Proxy Address are left blank. If
you have a problem here - ie it won't let you leave it blank - just
click into the field delete whatever was autofilled, then click your
browser back button twice, then forward twice and the field's blank!
bit Heath Robinsonish, but it works!
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Or read the page here
http://www.swissipcom.ch/pages/index.php?id=407 and amend to your settings
Hope it's of some help