Air carrier pilot reported being advised by ATC that they were not complying with the BOSSI 6000 ft. restriction after being cleared for the ILS 17C approach to DFW. Reporter suggested the glide slope calibration be checked.

Narrative

We were on vectors to an ILS approach to Runway 17C outside of BOSSI. We received a clearance to turn to our intercept heading; 6000 feet until established; cleared ILS 17C. We pushed the Approach button; and intercepted the Localizer outside BOSSI; and then the glide slope captured and the airplane started descending. It was a routine and uneventful approach and landing. After landing; we were informed that DFW TRACON wanted to talk to us. We called and they said that he was not calling to say we were in trouble or did anything wrong; but wanted to ask us questions about if our FMS was updated recently. He informed us that our aircraft was lower than 6000 feet when we crossed BOSSI. He said multiple aircraft had done it too that day. He just picked us to talk to about it. He said it was affecting multiple airlines and aircraft types. I told him once we are cleared for the approach and we are established on the glide slope that the aircraft autopilot stops looking at altitudes and starts descending when cleared for the approach. Our clearance to maintain an altitude until established on the localizer was followed. Being on the glide slope should mean we will make all of the intermediate fixes (unless there is a note on the chart that says they aren't coincident.). We discussed it more; and it was suggested that maybe the glide slope wasn't properly calibrated and maybe was projecting somewhat less than 3 degrees.Everything was followed per SOPs. I don't have any recommendations; except to maybe follow up with ATC if the ILS equipment was working right.