Cell phones that receive a call or SMS while near the laptop (Dell Optiplex GX520) will cause the computer to go into suspension mode. Read more about it in this blog, with additional comments in this article. Cheers Olle
The 520 is just an example. According to Dell this "feature" may very well be present in other models as well. If you have a 620 and a cell phone, why not go ahead and test it? /Olle