Does this count as Trademark Infringement?

I received a letter from Apple saying Suntex International, Inc has filed a complaint against one of my applications. They list several trademarks (U.S Reg. No. 2448022, 1718364, 3223059, 2469915, 3033720, 1718954, 3479453), but none of them seem to be related to my app. They claim I had unauthorized use of their trademarks for my products, but the only thing I used that is related to their trademarks is the number 24 (my app is called Solve 24). I based my entire application on an old Chinese game from more than 50 years ago. Furthermore, on the app store, there are at least 10 other apps with variations on that name as well and they seem to be doing fine as well. My question is whether this is an actual trademark infringement? Also, how should I respond to their lawyers? Thank you