I have an alternative suggestion for you! I have a nearly identical setup as you with the exception of a happy wife and my C475IP on the external network.
I use voipcheap.co.uk for all outgoing calls and my sipgate.co.uk local number for incoming calls. We have 4 phones spread around and I don't see the advantage of having them linked to lmce.
With voipcheap I pay £12.50 (I think) which gives me 90 days of free calls (except mobiles which are around 3.5p/pm) after which time my credit is slowly used up at very low rates. I have the credit automatically top up when the balance drops below £1 and voila .... 90 more free days. We're not big phone users and our £12.50 lasts us over 5 months.
Because the phone is attached to our virgin router it never goes down and always just works. Including multiple outgoing/incoming calls at the same time.
Just attach the C475IP to your external router and forward the relevant ports to the phones IP address. I know this doesn't strictly answer your question but I think it will solve your problem!