Note: Currently this device (Holovision Vio 2/Barix) does NOT work in a hosted environment. The device is missing an "Outbound Proxy" setting.
The field the device needs is a field called Outbound Proxy. The outbound proxy would then be set to sip.onsip.com.
The other fields:
SIP Server = domain
SIP ID = username
SIP Password = voip password
Authentication ID = Auth. Username
Here's what a REGISTER looks like without outbound proxy and with SIP Server set to the IP address of sip.onsip.com and a username of "userid":
REGISTER sip:188.8.131.52 SIP/2.0
Via: SIP/2.0/UDP 192.168.98.49:5060;branch=z9hG4bK6121180E
CSeq: 1 REGISTER