Most VoIP providers charge monthly fee based on the number of extensions for business accounts. The monthly rates are from $9.99 to $29.99 per extensions based on the company size and the phone features. The bigger the company size is, less charge per extension.
The Extension-N VoIP charges only based on the phone numbers by a very low rate of $3.99/number. The extensions are unlimited and totally free of charge. That will save hundreds dollar for the extension fee every month, especially for mid and large size of companies.
If you are using the VoIP system, all unlocked devices can be used on the system of the Extension-N VoIP. All you have to do is just to open an account, change the setting on the devices, then make your business calls.