Thanks for all the replies.
The FreePBX instant are provided & "managed" by a telco we use and we are potentially moving away from them so that won't stay. We do not currently have access to End Point Manager due to its being a managed FreePBX platform.
Also looking at the next level that involve muti-country and all that "UC" stuff (to some degree, more like using a VOIP App).
I'm just arming myself with as much info as I can as I won't be the one making the call on this. Other non technical factors has also come into play so looks like we are going to trial RingCentral.
We are Australian based so Scott's company won't fit into the equation. I also now found out that another of our little subsidiary have been using RingCentral so that gave them some bonus points just because.
I'll report back here if we do go with RingCentral and how things goes.
BTW: It turns out that we don't actually need SSO for any technical reasons at present and it was just thrown in there just because we are on the train to auth everything with Azure AD.