Is the following link of any help :
Details for configuring ZyXEL 2602HWL-61/61C Router - Whirlpool Broadband Forums
Is this device operating on a private IP address?
Does it support a STUN server ?
Perhaps to verify your voxalot registration information, you can try registering using a Softphone first (Eg. XLite) where the settings are well known and posted..