Seems to be very unreliable of late - using both a Siemens Gigaset N300 and Zoiper on Android.
Dialling certain numbers fails with "Network Busy" and others on the same exchange code dial just fine... I've noticed it on and off for some time - but as the target number is the New Spice curry place, I've just got proper annoyed.
Call quality seems highly variable too...
Tempted to move to Andrews and Arnold VOIP unless anyone has a better suggestion?
VOIPs only any good if it is actually reliable...