I have an Cisco Call manager 5.1 @ site A and one CUCM 6 BE in other site B.
Can I integrate both sites?. I read something about the BE is not appropriate for branch offices...
There is a WAN Link between A and B.
Yes, you could.
BE will support up to 20 remote branches, and also support SRST.
With BE, you would only have one server, so you would lose server and voicemail redundancy. Because it targets the commercial mid-market, you would also be limited to 500 users.
You could otherwise configure an intercluster trunk to link the two and maintain the CUCM 5.1 and CUCMBE on the other.
Hope this helps,