Welcome to the forum Rob.
Had this problem when I first attached a Sigma lens to the A700. Turned out that I needed to change a setting in the menu section. You should have an "Error Message" section in the manual that came with the camera... check for the error message and it will tell you how to reset so the lens can be used.
It's somewhere in the COG wheel menu setting for the A700 #2; RELEASE W/oLens is probably set to DISABLE - change to ENABLE.