How to enable AD schema user details updates between two sites / forests

My company has a requirement for updates to occur when users connect to the Intranet and update their telephone, location details etc which would then be replicated from one DC in an AD site / Forest location (B) to another DC in another AD site / location (A).

The DC in location B is intended to communicate to the DC in Location (A) via VPN and the location (A) DC would be located behind a firewall.

I would like to know how this could be achieved and what issues / recommendations anyone may have regarding this.

We are looking at MS Federated AD service but this would be investigated at sometime in the near future.

Thanks
M