oleg: Probably the reason no one mentioned Syslog is because it's something few people understand. For example, how would you capture the output of Syslog, convert it into something useful, and display it on a Windows or OS X based machine? And also, what I was really wanting to monitor was calls being sent to the phone port, not necessarily all SIP traffic (particularly in a case where a device does double duty as a gateway of some kind). I guess my main question, though, would be how do you capture the Syslog output in a Perl script or something of that nature?
QBZappy: An interesting suggestion, although it wouldn't appeal to me personally because of the "software bloat" involved - if all I want is a popup, I don't really want to be running a program that includes all sort of other functions I'd never use, and particularly one based on Java (I don't know why, but on my Mac Java-based applications seem to load very slowly and consume a lot of system resources). And as I said to oleg, I'm not looking for SIP notifications, what I want is notification of calls coming to the phone port, whether they come in via SIP, Google Voice, the OBiTALK network, or the Line port.
To all: I'm not saying that these suggestions wouldn't be useful to anyone, but unless I'm missing something here, they're really not what I was looking for. The script I posted yesterday is pretty much what I'm looking for, except I'll need to remove the 20 second timeout, and remove the print statement and substitute a system call to whatever command line program invokes a popup notification (notify-send on Linux, growlsend on OS X, etc.). Although, if I knew of a way to actually receive and process Syslog output in a script, and if it actually contains information I could use to tell what is being sent to the Phone port, that might be more efficient than polling the device every second. But at the moment I have no idea how to do that.