You can use an online calculator to determine the number and bandwidth of your Trunks, but do make sure that you factor in the nature of your business. Your Grade of Service (GoS) that calculates how many times a caller reached your company successfully, needs to be high if, for example, your business is totally administered and defined by a telephone system. In general, bear in mind that the more employees you hire, the less trunks you will need per number of people. This is because the higher the number of employees, the lower the chance that a larger percentage of them will need the phone concurrently.