If you are trying to dial this by turning on Direct Dial in the IVR, I don't believe that will work for ring groups. Looks like Direct Dial is more limited, maybe to just extensions?
If you are creating options In the IVR Menu and this is failing: under Advanced, check the Digit Length setting. For example, if you add an option 12345 to dial the ring group, but the Digit Length is set to 4, it won't work.