Hi,
Tough to say with any certainty but a couple of things come to mind:
1. Was it just the url changes or did you change content as well? The old urls look more 'optimised' (such as it is) for queries like 'calling xxx from yyy', the new ones look more like 'xxx phone cards' so if content changed along these lines at the same time this might be an area to look at.
2. That being said, a quick look at your backlink profile shows a high number of 'phone/calling card' exact match links and there was a google update around the 5th of October which might have got you, especially if you changed your urls just before that to include 'phone card xxx' in the url.
As long as the 301s were working properly there is no real reason for you to lose traffic just because of the url change. If your traffic dropped in early October, I think the update might be the more likely cause.