Networking two Unity Cluster to make a larger cluster

[iptuser55]

I`m need to network two Unity Connection 10.5 clusters together against a single CUCM so we create a larger user base due to the size of our company over 20k users 

I located info regarding the networking and that if you dont have any FW`s between the two clusters then you dont require a SMTP relay from my understanding so the networking is simpler. The reason is so that we can provide a single External DID number rather then one per Cluster

Unity SCCP integration 

Question

Do we do the following 

User A VM  Profile A- User is configured in Unity Cluster A

User B VM  Profile B User is configured in Unity Cluster B

So internally if someone the users below each VM Profile directs the caller to Unity A or B

Calls User A , CFW to Unity A

Calls User B , CFW to Unity B

My thinking that i set up a single VM Profile A

Add to the existing VM Hunt grp all the SCCP Ports for both Unity Cluster but use a single VM Profile and single External Number

Now we externally call in the single VM DDI  to listen to our VM box , User A is on Unity A and so the call stays here, now another user B calls in to Unity A, Unity A recognises that User B is not on Unity A and so requests if a user is on Unity B and so `transfers` the cal to Unity B 

Internally if we keep a single VM ,so that a user C calls User A , CFW using VM profile A , call is answered in Unity A, is user D calls User B ( configured in Unity B) , CFW using VM Profile A, calls is answered by Unity A  but ` transferred ` to Unity B

Is this correct, best way ?

thanks

[Jonathan Schulenberg]

Setup a voicemail profile and unique access number per-CUC cluster. If users want to memorize one external access number, that’s fine since it will be a small minority of calls; however, the vast majority of calls should be routed diect to the correct cluster. Cross-Server Sign-In and Transfer has a noticeable delay, prompt aplayed to callers, and is way more complicated on CUCM/CUC. It should be an exception, not the rule. For clarity, I’m not discouraging you from configuring it though.

PS- Unless you need MOH during supervised transfers by CUC, you may want to consider migrating to a more modern SIP integration to CUCM instead of SCCP.