Best Telephony Integration? Anyone happy with theirs?

Best Telephony Integration? Anyone happy with theirs?

Hi,

Looking for some input into telephony providers that integrate into Zoho CRM.

Ideally I want users to stay within the confines of the CRM. I don't want another system sitting on top of it.

Requirements
- client phones, it pulls up the client account 
- ability to transfer call and data to another user.
- ability to add a task to a call and relate it to a case, sales order or quote (or the call be related to same)

Anyone out there have a good telephony system they can recommend based in the US?

Thanks in advance