[Serusers] Linksys PAP2 sending 404 just after 180

Hi folks! First of all apologies if this mail shouldn't be targeted to this list but since this is full of VoIP gurus, may be somebody can help me (if anybody knows which is the most convenient list, please, point it to me). I've got a problem with a Linksys PAP2 (3.1.3) behind NAT. Both the ATA and the NAT router (not yet found out vendor/model) are in a customer's premises and I haven't got access to the internal network.... I've got two users configured in the ATA registered with received IP=NATdevice and two different ports. When somebody calls one of these users, I got 180 Ringing from one NAT port and after a few seconds I got 404 from the other port with a different To-tag, which I guess is because the router/ATA is somehow mixing users and doing some kind of forking.... SER receives the 180,404 sequence and relies back the 404, which makes impossible to call any user. This client has another ATA and both of them offers the same behaviour. The customer recently shut down the ATAs and probably there's something misconfigured....but I am completely new to the f#@#ng ATAs and I would really appreciate any hint about which of the config options of the Linksys can affect this behaviour. Thanks in advance, Samuel.