@Michiel: Welcome to vCampus. Are you referring to the ESPA 4.4.4, the serial data interface for paging equipment? PI Notifications allows creating Delivery Channel plug-ins to virtually send notification to any type of device or software.
Do you have access to an ESPA-ready API? If not can you make your own? You will certainly need to have a computer/gateway to route your message through a modem (ESPA capable card) by using the ESPA protocol. Is that the ACom installation you are referring to?
.NET Framework allows programming against serial ports. You will find several resources on the Web on the subject if you decide to go that way.
I am wondering as many telephony companies offer a paging service through Web Services, why you don't use them. Could you elaborate more your use case and shed some light?