/16 is all that fits in the x.x of 10.120.x.x. and give you 65,536 adresses (so 10.120.1.0/16 is the same subnet) use 10.119.x.x/16 for voip or 10.120.0.0/24 and 10.120.1.0/24 (/24 = 256 adresses for each subnet) or 10.120.0.0/23 and 10.120.2.0/23 (/23 = 512 adresses for each subnet) https://en.wikipedia.org/wiki/Private_network
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