If you already have a telephone system in place, the most economic way to link few offices is keep the existing phones, and setup external VOIP boxes.
There are many on the market, from the outright very expensive (several thousand per site), to few hundred dollars per site.
All depends in how many calls do you need at the same time on each location.
If you could live with two calls per site at the smae time, your options are numerous.
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