I have also noticed Gizmo having problems when calling SIP URI's sometimes....
When calling via SIP, try calling as *010xxxxxx@sipbroker.com instead of
xxxxxx@voxalot.com or
xxxxxx@us.voxalot.com (or the appropriate eu or au prefix)
Another possibility, especially in the cases of the PSTN gateways that seem to not function properly is to get a SipBroker Alias for your VoXalot account.
There is of course the option of forwarding FWD to VoXalot (try both
xxxxxx@voxalot.com, and *010xxxxxx@sipbroker.com) and calling FWD account through gateway
Then there is the SipNumber/FreeDigits forwarded to
xxxxxx@voxalot.com (putting the us, eu, au extension seems to work better) or *010xxxxxx@sipbroker.com