Xbee connection lost in crowded place

Hello there;
We take part in a competition where we have to read the values such as temperature and voltage. We use two Xbee Pro S3B. Last year, even in a very long distance from our workshop communication without any problem, while we go to the race place, although the receiver and the transmitter side by side, communication was broken and we rarely could exchange data. The frequency we use is 900Mhz.One GSM operator is use this frequency band in our country. We think that GSM operators are pushing our frequency. I’m curious about your opinion.