If they don't offer a voluntary, I've been told by a couple of people I trust that if you make it known you'd be willing to go when it becomes clear a layoff is coming, they will usually accomodate (even if a voluntary option is not offered). Either way you get the severance. Involuntary is actually better because you get paid for some extra time to look for another internal job and you're eligible to collect unemployment payments if you want to do that. The last thing ATT will want to do is send any kind of signal to Wall Street that they're having any trouble paying their massive debt, which is how an announcement of a large layoff might be construed. Instead they'll continue doing rolling org specific layoffs throughout the year that they can keep quiet. Timing is impossible to predict, but you can be sure you will get your chance to go in 2019.
Bumped from @WPSoLHr-cjb for info.