Is there a 7499 or a 7501 that he could've meant to dial instead? I'd think that number would be pretty isolated so that nothing was hit by accident.
Flight controllers instruct pilots to dial a certain code in the transponder to identify them from other traffic. The code varies from plane to plane and controller to controller. At least in private planes, pilots are trained to dial the last two numbers first then the first two numbers in order to avoid accidentally dialing one of the emergency codes. So, in your example, a controller could give the pilot a 7501 transponder code. The pilot would dial the 01 first to avoid the possibility of accidentally dialing 7500.
>>Is there a 7499 or a 7501 that he could've meant to dial instead?
Yes, if he was tapping it out in Hand Jive.
7501 yes, 7499 no. The digits of the transponder only go up to 7. Therefore, 7777 would be the highest number the transponder can go. 7500 is hijack, 7600 is nordo and 7700 is emergency.