If you are ready too, you could install an linux machine, and set up a shaped interface on it, to reduce it's max speed, but this would not be enough.
My father implemented VOIP phones over vpn between Austria and Germany some years ago, and some handheld where used too.
He told me that the problem was not the bandwidth, but the latency.
Over an EDGE network, you have easily almost 300ms of latency, and this only makes every thing harder to a networking application, as even if the theoretical bandwidth is ok, the latency could be too high to have something responsible.
The problem is, that you cannot really "forge" high latency over a network and materials that are designed to have the less latency as possible...
So, apart the traffic shaping modules for linux, I don't know of anything else that could help you.
PS: looking for links about traffic shaping, I just found netlimiter: http://www.netlimiter.com/ which is a window application that
NetLimiter is an ultimate internet traffic control and monitoring tool designed for Windows. You can use NetLimiter to set download/upload transfer rate limits for applications or even single connection and monitor their internet traffic.
Looks like a traffic shaper for windows to me. Being exclusively on linux at home, I have nothing to test it.
http://sysblogd.wordpress.com/2008/05/14/traffic-shaping-linux-restrict-bandwidth/ which leads to http://lartc.org/wondershaper/