It looks like the typical double nat problem, unfortunately there is now one answer as there are numerous types of NPAT and each needs to be able to traverse all RTP and SIP signaling correctly. I quick fix might be to use ports other than 5060 for each of the SPA's (and expand your forwarding rules to suit) the audio then might or might work also, depending on how you set up each router, particularly the remote office router.
Wellllll it turns out to be a missing configuration in sip_nat.conf. AS it was missing asterisk didn't know what was my wan/lan networks.
It work fine now! at least with one linksys SPA-921 each one at a remote location.
I'll post tomorrow with the two phones in the same remote location.