I too just returned a rent a car from Enterprise on June 9th.. got call today and did not pick up.. now I am glad I didn't. Maybe I'll answer tomorrow and find out what they want :)
I'll let you know what I find out

Just got a call from this number on my cell phone (8:15 PM on a Sunday night). Didn't pick up since I didn't recognize the number, and it was on my personal cell. I did use an Enterprise car one day this week when my car was in for warranty service... my dealer paid for it, since they usually offer a loaner but they occasionally use an Enterprise vehicle when all their loaners are in use. Thanks to everyone who left the info on this site!

got a call from this number Sunday June 12, 2011 7:50 CDT.  just rented a car from Alamo and returned it Friday June 10, 2011 about 7:45pm.  

as someone else wrote.   looks like Alamo and Enterprise use the same service.

Received call from this number on Monday at 11PM, so I did not answer it and no voice message was left.  Rented a car from National and Alamo over the weekend.

rented a car from enterprise last saturday and turned it in on wednesday. just got a call from this number and didnt answer it. they didnt leave a message