Fortigate 60E with a 4G bridge modem - Private addressing

Good afternoon everyone,

I have been tasked with getting a remote site up and running with a 4G solution since there is no fixed line broadband at the site.  The idea is to get a vpn back to head office working by having the remote 60e configured as a dial up vpn client back to our main fortigate fw.  This has been tested in our office and the tunnel routed the data traffic no problem.  

My issue right now is with the addressing.  For the data network at the remote site its not a problem as its using a 172. private address.  My problems is that our phone system uses a 10. address schema and my l3 switch on site uses a small 10.255 netwrk between itself and the fw.  When i look at the wan interface on the 60e the 4G network is giving me a 10. address but with an 8 bit mask over it.  That blocks out me using any of the private 10 range on my internal network.  For the small network between the l3 switch and fw its not really a problem as i can just change that to a 192 address on each side.  My real issue is what i do with the 10. voice network.

Thanks in advance for any replies.