We had to disable summoning (Note: In ZA) yesterday via emergency hotfix due to some exploits. We're hoping to re-enable summoning as soon as possible. We know it's a big inconvenience and we apologize.

On a related note, we're looking into fixing some issues with summoning that would once again enable us to allow warlocks to summon people into an instance from anywhere in the world. This could happen as soon as the next big content patch (2.4.0) if all of the technical issues can be resolved by then.