Best Unified Communications system for integration with Salesforce

I have a client running a call center that’s looking for a Unified Communications solution that will integrate well with Salesforce.

The list they have come up with already is 8x8, Avaya, 3CX, and Mitel (ShoreTel). Has anyone had experience with any of these?

It would be great if I could host something and provide what they need, but I don’t know of any self-hosted PBX options that have great Salesforce integration.

I am not aware of any that are local ones.