I'm working in a company that developed a softphone integration, so I'd be biased with my answer. Nevertheless, as you've mentioned that your client needs collaboration, I'd suggest you try Ringotel Shell as it stands out from other softphone solutions providing advanced collaboration capabilities, like personal and groups chats, public channels, files/photo/audio messages/location sharing, video calls, etc. It is also focused on security encrypting chats, files and calls, registration via QR codes and much more. Our service is subscription-based, but it includes all the features out-of-the-box, as well as iOS, Android, Mac, Linux and Windows apps, and great customer support!