Can someone recommend a good modem to use with notepager pro for Orion NPM alerts?
I haven't tried using a modem directly for alerting but I've had a lot of success with using a 3rd party SMS gateway provider.
Clickatell - SMS Gateway - Alert Overview
You could also accomplish the same thing with push notifications through Slack for free.
Slack Alert Integration - Overview
NotePager Pro version 5 can use any cellular device that can install as a modem in Windows and accepts the AT command set for messaging. We've worked with Verizon and AT&T Aircards nearly as often as the more industrial Multitech rCell 100 or AirLink GX450. As a personal opinion, I've found Multitech's hardware to be the most reliable from what our clients have reported in real world use and I'd recommend taking a look at their product line.
I helped a client install the Multitech rCell line of modems for their PageGate setup. Was a bit of a pain to get through the mire of the documentation between the hardware manufacturer and the carrier, but once we got them communicating it was a pretty straightforward process. Took about 3 hours start to finish.