Booting Phones Through VPN

I’d like to boot my Polycom Soundpoint 501 phones in the branch from the Asterisk server in the main office. I am able to ping/connect to the server in the main office from the branch office, but when the phones startup I get an error that it cannot find the boot image or connect the the boot server. Is there anything I should be aware of that would prohibit booting via VPN?

Are there any other, better, strategies for setting up a distributed phone system?

Thanks!

Does the vpn pass tftp boot requests ?
and are the boot server addresses correct, I guess they are being given by DHCP and is this correct ?

Ian