This might not be quite what you are looking for, however it might introduce some interesting possibilities. Take a look at Edgewater Networks, something like the Edgemarc 4200 series. It has QoS, a rock solid firewall, a VPN, a ‘proper’ DHCP server for the phones. I don’t think it has the Dynamic DNS Support though - but that is easily enough handled by the Asterisk server and a proper daemon (although it might have it?). Just a thought - it’s purpose is beyond what you are looking for but …
p.s. - if you are just looking for a SOHO router, Dlink has one that does all of what your are looking for I think:
you don’t need to use the second WAN side. I have no experience with either but it provides some options.