Almost all of the telephone providers today use IP over voice technology, including Bell. The reason behind this is the lack or little infrastructure change external to your facility necessary to support such a system and the additional services available within the system.
Basically, this is a system that utilizes the internet to transmit and receive phone calls. Because the infrastructure for the internet is already in place, both external and inside your facility, telephones can be added with little or no internal updated wiring within the office structure. Any office wired for the internet can install IP over voice telephone systems with little or no changes to the wired structure within the complex.
It is possible that the existing computer or server system may be insufficient to handle the additional load of telephone traffic your facility may receive. Evaluations of the amount of traffic your office receives via the telephone must be properly evaluated prior to installing the system so that the amount of equipment and bandwidth necessary to post this system is in place.
One issue that may well determine the decision to go to IP over voice telephones should not be made strictly on cost. It is important that such determinations be made prior to the decision to implement IP phone systems so that expectations can be met with success. Keep in mind that if an improvement in your computer power within your office space is necessary this may also be an indication that it is otherwise inadequate to meet other needs as well. Such evaluations must also be included in the of the amount of bandwidth necessary to support the telephones.