Ask HN: VOIP suggestions for e-commerce site?

We're an e-commerce company with a small-sized call center looking to replace our traditional Nortel PBX phone system with a VOIP solution.

I was wondering if anyone had any positive experiences with certain companies or any key features/issues to watch out for. Ideally we'd like to integrate it with our order management system via an API too.

Any suggestions or pointers would be greatly appreciated!