Take a fools advice — turn off SIP helper, at least in the current stable builds off the main TomatoUSB website.
It is possible that some of the more recent RAF, etc. builds may include newer versions of Conntrack, etc. but the early version DEFINITELY causes problems in certain cases.
Specifically, if you are using PhonePower's SIP Trunking option with an Asterisk server, then SIP helper will corrupt the destination of inbound RTP audio packets destined for Asterisk. The end result is that you get no inbound audio on incoming or outgoing calls. It passes all the way through the router, through the firewall on my Asterisk server, and then gets basically chucked as an invalid. If I turn off SIP helper, everything works just fine.
With some of my other trunks (SIPStation for one), SIP helper hasn't caused me any problems that I am aware of — but it hasn't helped in any way I can tell either (i.e. it works just as well with it turned off).