One of the things I (and support) found out is if you have specialized rings on the Grandstream, one of them (5 maybe) is silent, and does not ring the phone. I just had swapped out my Linksys because of repeated problems, and this ring is what I used for my wife and my cellphone so my kids would know it was "safe" to pick up the call.

When I was testing with the new TA, it would never ring. Took support quite a while to figure out the issue.

Don't know that this is your issue, just throwing it out there.