The UCM series of IP PBXs features 2 model types: the UCM6100 series, which consists of 4 models designed for small and medium business users by supporting up to 500 users, and the UCM6510, which is geared towards small and medium enterprises by supporting up to 2000 users. Those looking for support for E1, T1 or J1 networks will want to choose the UCM6510. These award-winning IP PBXs offer an advanced set of features with no licensing fees, upgrade fees or costs per feature.