For most ppl, it's not necessary to place the voip adapter between the modem and wireless router. The voip provider typically suggests it for their own benefit, not necessarily yours, since it reduces tech support calls from those few ppl w/ issues. So unless you're one of the unlucky few (even then, a good router w/ QoS options can usually help), just patch it to the switch like any other wired device.
[wireless router]<-- wire -->[voip adapter]
If you want a "wireless" voip adapter, you can convert it just like any other wired device using a wireless ethernet adapter (aka, gaming adapter).