This router has a feature called "SIP ALG", and you will need to disable this for a sound and robust VoIP service from any VoIP provider. Just following the following steps.
1) Make sure you're using a computer that's connected to your network
2) Open a browser (Internet Explorer, Firefox etc) and type 192.168.1.254 or businesshub.home in the address bar. You'll see the BT Business Hub home page.
3) Click on ‘Advanced Settings’
4) Enter your admin password (unless this has been changed, the default admin password can be found on the bottom of the Business Hub 5 or on the plastic pull out on its back).
5) Click ‘Continue to Advanced Settings’
6) Click ‘Firewall’ then ‘Configuration’ and scroll down to ‘Application Layer Gateway’
7) Click Yes next to ‘Disable SIP ALG’ and click ‘Apply’
SIP ALG will now be deactivated on your Business Hub 5.