Yes your ATA 186 will do VOIP with compression. Most likely it it doing it already.
The trouble is that with only 64 kb, any aplication that you try to “share” on the same pipe with voip will take all the existing bandwith, not leaving enough for the ATA to provide a good quality call.
If you have good routers, you should be able to make what is call “bandwith reservations” on a intranet to make the voice trafic a priority over normal data.
But when you go to the internet over that narrow bridge (64 kb) your data will bump your voice over the rail and into the water bellow.
You could do only so much with such a low bandwith.