I may know the answer to this problem, as I struggled with something similar for months. The good news is I think it has nothing to do with Apple products, and only an insurmountable characteristic of AT&T's infrastructure. I promise not to get too technical. I don't know those details anyway. I had U-Verse voice, and my wife and I had an iPhone 4s and a Samsung flip phone. We used "Locate Me" to forward incoming calls coming to the U-Verse line to both wireless phones. Most of the time this worked great, but occasionally the phones would all ring once, then direct the call straight to voicemail. Also, by the way, we were using a single U-Verse Voice mailbox, shared no matter which line was called. In my case the U-Verse people kept blaming the AT&T Wireless people, and vice-versa. Eventually, I noticed a pattern. The only time the calls went straight to voicemail was when either wireless phone had run out of battery, and shut itself off, or when one was otherwise turned off intentionally. It turns out that the way the AT&T network implements this feature is incapable of ignoring the minor inconvenience of a single device being turned off, and therefore unavailable. This is of course the way users would like the network to behave - simply allowing whatever devices are still on-line to ring as normal. Unfortuntely, if any device is off-line the network perceives this as a "device unavailable" condition, and follows whatever protocol is currently configured for that condition. In my case, after ringing once the call goes to our U-Verse voicemail. Other responses are possible. The other good news is that there IS a work-around, imperfect, but not bad, and my family has been using it for years. Just make sure you keep all wireless phones connected to "Locate Me" charged and powered on at all times. If you ever see the problem agaIn, just look for the phone that is powered off, or with a dead battery, and fix it. The system will work perfectly again. By the way, I reported my finding to AT&T, so I think there may be a handful of engineers on a level 3 support team somewhere who understand this problem, but I imagine it remains undocumented, because AT&T would rather let customers suffer than allow the company to suffer any embarrassment over a disfunctional feature for which they have no solution - just my opinion. I hope this helps.