asterisk users - Sep 2005 - Slight OT: Multi WAN Router and SIP Calls

We are drafting a plan for a new office setup. The users will be using Cisco
7940 phones registered to a remote Asterisk server. We were thinking of
using two ADSL lines coming into a Multi-WAN router to allow for load
balancing. As opposed to setting up half the users on one ADSL line, half on
the other.

It would seem that this could cause a problem where a given phone could be
registered out one ADSL line's public IP address, then that line becomes
saturated and the user tries to make a call that is routed out the other
interface. Wouldn't the remote asterisk server refuse the connection since
it is now coming from a different Public IP address?

It sounds like it would cause an issue.

Thanks!