I note that some users report successful set-ups not using an outbound proxy setting. On the Gradwell support portal I found the following under the heading Standard settings for most SIP Phones/devices:"There will be no need to use the outbound proxy if your adaptor/phone has a static public IP address assigned to it."
Does this apply when one is using the VoIP trial service "sip2.plus.net" hosted by Gradwell if one is using a Plusnet account with a static IP address? In which case would the reliability of the service be improved if the outbound proxy setting was also "sip2.plus.net"?

This message applies to almost all SIP services and means if the device you are using has a public IP assigned to it directly.

For the majority of our customers, regardless of having a Static or Dynamic IP, this will not be the case. This situation requires that you have a block of IPs or have a router that will transparently push the data into your internal network (The X-Modem is the only one I know that does this).

One exception to this is customers that have a modem/router with a built in SIP client, such as the Draytek Vigor 2600.