These are called 'Ghost' calls. They are made directly to your SIP phone over the Internet.
These calls do not come from our system, but from scanners on the Internet targeting blocks of IP addresses at random.
Luckily it is possible to overcome these by changing a setting on your handset.
You will need to login to your handset's web interface, and locate its IP address on your local network and then change the local SIP port setting from 5060 to 5065.
If you need any assistance, please call our Support team on 0800 160 1111 and they will help you with steps to update these settings.