Error q.850 cause=17

[Nomads]

Hi All.

Have strange problem. User with extension 807 is unable to call extension 814. Error q.850 cause=17 is displayed on the phone. All the other phones (same model) can call the extension without any problems. I changed the phone's assigned extension 807 to 812 (spare) and now she can call 814! Have checked the phone settings and extension settings and cannot find any reason why extension 814 is blocking(dropping) incoming calls from 807!
Has anyone come across this problem before?

Thanks
Rob