Sometimes, after troubleshooting an issue, it turns out that it was the result of a simple mistake on the user's part - perhaps they overlooked something, or chose the wrong option by accident. Something I am still working on is communicating to them what the problem was without being too blunt.
For example, just now I was troubleshooting an issue where a user's VOIP phone wouldn't ring. After calling to confirm that it went straight to voicemail, and that it wasn't a problem on my end, I went to take a look at the user's equipment, which all seemed to be in order, and was capable of making outgoing calls. I experimentally tapped the Do Not Disturb button a few times... and realized that it might have been on when I started, so I called their phone and sure enough, it rang.
And then I said out loud, to the user, "I, uh, think you had it...