+1 I've had great success with voip.ms as well. Let's also not forget that with voip.ms if you're going to be in a particular region of the world in advance, you can move your gateway to a closer POP. i.e. you're normally in the US, but plan a week trip to the UK, you can move your connection point from say Dallas to London.

Being closer to the POP reduces the chance of having quality issues (less people in between you and the gateway). As far as low bandwidth, a provider and client that supports G.729 should work well. I've regularly used a softphone on my cell phone (Bria) to access my Corporate PBX (US-Based) from UK and Netherlands with G.729 over a carrier 's Data Network. (3 and Vodaphone)