SJ Phone comes with a default profile called "SIP Direct". Once the Plusnet settings have been put into a new profile the SIP Direct profile seems unnecessary. I deleted mine as I could not work out any use for it. The SJ Phone still works but I'm wondering what purpose the original profile had and whether there would be any reason to keep it. I'm also getting ten minute cut offs sometimes now. Could that be affected by the deletion of the SIP Direct profile?
I'm not an SJ Phones user, so this is slightly guesswork, but as no one else has answered yet, I'll have a go. I assume 'SIP Direct' is the default profile that enables you to make calls direct to other SIP phone users without you having to be registered on a SIP server or a PSTN gateway. You would need to know their full SIP address, either in the form fred@<theirIPaddress> or fred@<theirDomainName>. To call you, unless you have a domain name pointing to your computer, they would have to call <yourname>@<yourIPaddress>. That's OK if they have a softphone, but difficult if they are using a phone that will only dial numbers, such as a traditional phone connected via an ATA. If you are behind a NAT router that doesn't handle SIP traffic automatically, you would also need to set up the appropriate port forwarding rules. You would not be able to make or receive calls to or from the PSTN network. I think SJ Labs set the profile up for network users to give the phone a try within their own network, so they can evaluate it without having to setup a VoIP account with anyone. I don't think deleting it will have been the reason for your cut off problem.