VoIP - Calling With a SIP URI
Incoming calls can be made from the internet to sip:email@example.com and delivered just like a normal incoming call.
The format of your number should be in +44 format, eg to call AAISP support you'd use: sip:+firstname.lastname@example.org
You do need to use the hostname aa.org.uk as we use SRV records to direct the call, so using an IP address instead is not supported.
The CLI is not trusted for such calls, so are sent to your SIP handset with a ? on the front, and not passed on if the call is diverted.
So, for example, without any configuration or account details a softphone may call your A&A provided VoIP number, as shown in the image here.