If you want to spend soo much money on PBX system then go for Mitel, its the best for call centers, withh all the feature you want, but if you want all those expensive PBX feature then go for “Asterisk”, all you need is some technacally sound person who can configure it. its all free at http://www.asterisk.org.. do check the feature list of it and then compare to other PBX system.
feel free to contact for cost effective solutions for international call center.
Regards
Omer Ali