What is the best option for remote paging when network services are down? I'm looking for a solution that can let me know when network services are down during off-hours only, and especially when email/smtp services are out. Therefore, it needs to be redundant to our network and power supply.

I'm imagining a cellular modem is one option. What's the price range for these? Is anybody using them and feel that they are worth the cost? I'm imagining that it's something we would end up sending an emergency page ~ 1x/month at most, so I'd like the pricing to reflect that--I don't mind a high per-page cost as long as it has a low recurring cost.

Another option would be to expose at least one server to remote ping, and run a check script on a remote server. Are there paid options for this?

Currently, we run Nagios on a Linux VM on a Windows 2008 Hyper-V host. It would be great if the solution would work in that environment, but I know it's tricky with external devices, and we could move Nagios to a standalone workstation if needed.