I hope this is for fun and not mischief. If you do have evil intentions, you should know that officers often use cell phones to talk with each other or with dispatch. Also, with the 800 band system, most officers are able to make those calls from their radio.
Most of Story County runs on a 800 band system, so most scanners won't pick up these frequencies. Very few scanners will pick up VHF (like what 90% of the nation uses) and UHF (especially high UHF like what Story County uses), so you may have to get two scanners if you want to listen to surrounding counties. Story County does have some VHF frequencies that broadcasts dispatch traffic only, but you won't be able to hear the portable traffic.
The following website has some of those frequencies. For the most part, you will want to look under the
trunked systems link. I have to warn you that some of the channels are misnamed.
Story County, Iowa (IA) Scanner Frequencies and Radio Frequency Reference
Also, just like a radio or tv station has to post FCC info available to the public, so does dispatch centers. If you can't find a frequency, you can stop by the applicable dispatch center and ask to see the frequencies.