TerrorBlade: Kind of low pitched buzz when the corded phone is plugged in.
Also: Registration State: Can't connect to the login server.
TerrorBlade: Okay, busted out a newish ASUS router I won a couple of years back and the phone is working properly in about 5 seconds :/
So older dynalink routers are apparently incompatible with VOIP, the RTA300 definitely isn't at least with my config.
Thanks a lot to everyone who contributed and I hope this thread helps someone else out in future.