Not sure if it is jamming, but under capacity. That many people represents a lot of cell phones. Need to add a lot of micro cells to provide enough coverage. Calls can be ranked, so more important calls go through. Land lines have first priority, then existing cell calls in the cell, then cell calls handing off into the area, then calls starting up. If you know this, you can take advantage of it. During Katrina, I was able to contact my family via land lines.
I thought landlines and cell phones were separate systems?
Trump had more people at the RNC Convention than Hillary did at the DNC Convention. Has anyone heard of cell phone problems at the RNC Convention ?