Hi zain04,
There are two common methods. The first one is to use Floquet ports, which is slightly more complicated, but allows you do check off-normal incidence response. There is an example in the HFSS help. The second is a simple "waveguide" test for normal incidence response:
Surround once unit-cell with a solid air object, and elongate both directions normal to the surface of the EBG unit-cell (let's call this Z) by at least a quarter wavelength. Apply perfect E surfaces on the faces of the X axis and perfect Hs on the faces of the Y axis. Now, apply wave ports the the faces on the Z axis. You won't have to set an integration line, but do de-embed the ports to the EBG surface. This will simulate a normally incident plane wave on an infinite array of unit-cells. The Phase of S11 is then the reflection phase you are looking for.
Good Luck