Re: i am looking for a voip phone porvider for a call center
You didn't specify a country - so here's my answer for USA:
http://www.broadvox.com/SIP-Trunking-contact.aspx
They specifically speak to Call Centers and the like. Several other voip providers do as well - even several of the "pre-set" ones listed in 3cx.
I personally use NexVortex and they have a few products that they say can be used with call centers, phone card resellers, etc.