It's not easy backing up from behind the firewall.
What about using a service that will backup the mobile clients to an offsite repository that is accessible also from behind the firewall.
I was pitched something not too long ago about such a service, can't remember the name now unfortunately.
Otherwise you could look into some sort of WebDAV + Fuse setup or some specialized file system that is cached on the client but then syncs with the server in the background when available, then all your backups are local.
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