Out of band notification when UPS loses mains power?

The serial comms port is an indication of either an old or a small UPS (unlikely to provide you with the 45 mins of coverage for any notable number of server machines, network infrastructure and peripherals). It has been the state-of-the-art interface around 1995, but has been superseded by SNMP-capable Ethernet interfaces since.

Most of the available monitoring systems are capable of using a custom check or script to get the current UPS state over the serial port (obviously, it would need to be physically connected via a serial cable) or listening to SNMP traps and raising alerts and notifications in various ways - feel free to choose one to your liking.

If you already do have UPSes in place which cannot be upgraded with SNMP cards and have no other means of connecting to the UPS to monitor its status but are using servers with redundant power supplies, you might consider a setup where at least one PSU is connected directly to the mains, while the other(s) is(are) fed through through the UPS. Upon a power failure, you should get a PSU power loss event on the mains-connected PSUs, which you could forward and treat accordingly in your monitoring / notification system.

Note: if you are using your UPS as the single surge protection for your servers, you might want to add an external surge protector to reduce the risk of a potential PSU damage upon voltage spikes in the last scenario.

Most UPSs will include a rudimentary proprietary in house version of a notification package while others will co-brand a more commonly known solution, the way DVD burners include a stripped but functioning vesion of Roxio, your UPS will include something similar or at least a gnawed down version of APC Powerchute.

Regarding the serial connection, it is common and is typically connected to the RS232/serial port found in older computers and special-ordered as an offlist option in PCs today (harder than hell to order in a contemporary laptop) and the UPS sends almost primitive signals across the serial line to a listening/waiting software app that's captured the serial port for just that purpose.

Sometimes, the UPS has enough embedded code to be programmable with a phone number and the serial is jacked directly into an old external modem with serial cable between the PC and modem.

We prefer PCs as the monitoring agent and just deploy a serial/USB converter from any professional vendor (even Amazon sells them for less than $15 and it connects the serial to your PC's USB). We use GFI LanMon for this and other event tracking and its never failed us. I think we paid $7K for an unlimited GFI LanMon site license back in 2004 so I don't know much it costs today but they negotiate and their stuff suits us as a medium-sized company in six states.