set up servers to "phone home" to detect network outages

Been a while since I worked with Linux, and not particularly familiar with scripting (PHP or otherwise) on Linux, but I suspect it wouldn't be too terribly difficult to just have the Linux script ping whatever hosts you're interested in, and if enough ping's fail you can use sendmail to send yourself an e-mail.

If you're interested in an all-Windows solution, have a look at SpiceWorks: http://www.spiceworks.com/

I'm a fan of Xymon (http://sourceforge.net/projects/xymon/) for host/service monitoring.  It's designed to run on a unix/linux host, and while it can certainly be used for farily comprehensive monitoring, alerting and reporting, a quick install with a "bb-hosts" file populated with your Windows server IP address and names would help accomplish your goals.

However, for a quick-and-dirty solution that doesn't require installing and setting up Xymon, you might try running fping out of cron.
fping (http://fping.sourceforge.net/, available packaged for various Linux distributions) can specify multiple ping targets, and report just the unreachable ones:

fping -u < mylistofserverIPs.txt

Set up a cronjob to run this as frequently as you like, and the output will generate emails to you -- but no email if all are online.

The reason I want the servers to "phone home" to the monitoring script is because they are behind firewalls and I can't ping them.