It's hard to recommend to A-Z route with best quality and cheap. Most of A-Z provider buy-back all routes (include white and gray route) put in the mix and sell it to you.
If your customer want to provide calling card, the best thing is focus to 1,2 main countries -> Find white route for it and because they focus to 1,2 countries then their using minutes will high -> lower rate you can get.
The rest you can find one good A-Z route, mark up then sell (but don't expect low rate and high quality for all your destination
Also just curious, who's your DID provider in Westin? I'm looking and DID provider in seattle area also.