We use
http://www.splitinfinity.com and we're VERY satisfied. Not only do we get personal cell phone numbers of the techs, but we can also send text messages to their cell phones via Sprint PCS on the web. So if my server is down, I just go to sprintpcs.com and type in a text message to him and it's back up within usually about 5-10 minutes.
We rarely experience downtime and the longest we've ever been down was for about 20 minutes. We're VERY VERY happy with them and the price kicks ass too!