I am experiencing a similar problem. I am using the DIR-655 in front of a sipXecs PBX. For remote worker phones to connect to sipXecs, SIP ALG needs to be disabled. When I first started using the DIR-655 with the original firmware, my remote worker phones were no longer able to register. My first thought was that SIP ALG was enabled, but no setting was listed.
I applied the 2.01NA firmware, and the SIP ALG setting is now shown. This setting is enabled by default. I'm pretty sure that the original firmware had it enabled, as 1) sipXecs behaves the same way with 2.01NA with SIP ALG enabled as it did before with 2.00NA, and 2) when I disable SIP ALG, I am no longer able to register to my ITSP.
In any case, it looks like the DIR-655 rev B with the 2.01NA firmware is still messing with the data somehow even when SIP ALG is disabled, as I should be able to register with my ITSP with SIP ALG disabled. I can successfully register to the same ITSP using the same sipXecs box with two other routers that don't do SIP ALG.
If I have some time, I may run a trace to confirm it. However, I thought I'd mention this now, just in case someone else runs into a similar problem.