When you’ve been working with SIP phones for as long as I have, you can be forgiven for developing a deep-set hatred for “SOHO grade” ADSL router equipment. More specifically the SIP ALG that router manufacturers seem to love putting into routers these days. I’m yet to encounter one that actually works. In most cases these ALGs tampering with passing SIP traffic cause more problems than they solve. They are a nice idea but in reality they just don’t work.
To that end, my preferred router is one that doesn’t try to be too clever for it’s own good. One that has no SIP ALG inside it what-so-ever. For a small office on a normal ADSL connection I can’t recommend anything better than the Zyxel P660r-D1. It’s a basic single port router which will provide NAT and DHCP if you want it to.
For a home or small office using VoIP over ADSL you can’t go far wrong with one of these routers, a decent switch, a reputable VoIP service provider and decent quality phones (i.e. Snom!).