Is there a way, specifically on Microsoft Server, to supply two different call_srv ip's in DHCP Option 43?
No, not that I have found... You need to use option 125.
That said, you really don't need it... Once the phone connects to ANY call server in the cluster that knows the phone, it will tell it what primary and secondary ICP to use. You can even have everything connected initially to PBX A, and it will tell that phone that it's primary ICP is PBX B and secondary is PBX C and it will work fine as long as all 3 are properly clustered.
