Trunk line is a term that we inherited from the former technology. Then, it literally meant the number of phone lines you had. With two trunk lines, for example, you could hold a maximum of two simultaneous phone calls.
With VOIP technology trunk lines are more ambiguous. We use them as a gauge for billing purposes. Typically, we have one trunk line per three phones. Let’s say you have five phones and two trunk lines on our system. Two of your people are already on the phone and a third person makes a call. The third person will still get through. Likewise with the fourth and fifth. This is a benefit of VOIP. You will never get a busy signal, either in bound or out bound, with our system.
Basically, you can either make an educated guess, or you could obtain a detailed history report for analysis. Most businesses need about one trunk line per three phones. If your business has 10 phones, then you should probably get 3 trunks.