In the past I've bemoaned the fact that external VOIP services do not seem to work well for me. (VOIP within my own network is fine.) Recently I've been using Whistlephone and the quality is quite acceptable; no obvious dropouts. Whistelphone is a free service for calls within the US. It is ad-supported. You listen to a 15(?) second ad at the beginning of your call.
You must download their softphone application to sign up; however they do not prohibit (but do not support) use with ATA devices and/or Asterisk. I use it with Asterisk and my ancient 2500 set, so whatever they are doing right does not depend on the client. (I suppose it is possible that they are just closer to me in some sense than other services.) My only worry is that lately the ads have been for Whistelphone itself, suggesting that their funding model may be failing. :(
Dan Lanciani ddl@danlan.*com