Hi, We are now using BT landline in our office. There are 5 handsets for 6 staff to share in the office. Our staff works in three offices. From time to time one person need bring the phone and then walk to the other office to pass on. Now we want to upgrade our telephone to VOIP, any suggestions please. The following features are important to us. 1, A good tariff. 2, Intercom calls with coworkers 3,Auto call attendant 4,use the current number we are using. Thanks.