Hey all, long time lurker, first time poster.
I got the same phone call. Originally I thought it meant that I had an interview for funding, but not necessarily that I got accepted to the program, since the application processes were separate. Check this though: http://www.umass.edu/english/graduate/admissions.html
It says "Once you are admitted (and if you have applied for funding), then you will be interviewed by the Writing Program" which certainly sounds to me like the WP only interviews people who have been accepted.
You might be 3/3 Trip! And this might be my first admit!