if we are going to speak about “nice GUI” but open souce under the hood, then Porta is a great example of it. All their billing and softswitch are assembled from open source products 🙂
MINDCTI is a very, very expensive product. It is for large telecoms with millions dollars budget.
I would add the russian billing packages – IPSoft and ProperSoft. Both of them are just excellent and not too much costly.