For what it's worth, I just replaced the DIR-615 I had at my house for a number of reasons, one being problems with my switchvox extension. One of the problems I had was when someone would call my extension, I would "answer" the call, but I couldn't hear anybody on the other end. They could hear me, but I couldn't hear them. A flash hook usually corrected the problem and I could hear the caller on the other end. I didn't have this problem on every call, but it was getting worse and worse, and coupled with the other problems I was having with the router, I decided to replace it.
To the best of my knowledge, the problem I was having has gone away (and so have all the other non-voip related issues I was having). Granted, I just switched it out on Monday, but so far, so good.