Go to a PC on one end of the wireless link.
Ping another computer or device on the remote end.
If you have a delay of less than 500 ms round trip, and around 120 kbs of available bandwith, you could use any VOIP product that it is out there up to three or four channels.
I have done VOIP with DSL links that carry only 64 kb of data, with excelent quality.
As Mike pointed, one of the easiest options may be a Quintum, or a Sipura if all what you wanted is one or two phone lines across the link.