Registering a SIP Trunk refers to connecting it to a device, whether this is a dedicated phone system or an individual handset or smartphone app. It requires a username, password and SIP domain, once these are entered in to the device you should then be able to make and receive calls.
When you are logged into the Orbtalk portal and and viewing the 'SIP Services > Overview' page, each SIP Trunk will be listed. You will be able to view the trunk's username together with the SIP domain (sip.orbtalk.co.uk or sipgw3.orbtalk.co.uk). If you are unsure of the password you can change it by using the cog button to the right of the trunk. These details can then be entered into your device to establish a connection.
You can view more information about compatible devices on the Orbtalk website.