While I don't have the USMS frequencies for the Phoenix area, allow me to provide you with information that has helped me in the past. The USMS usually piggybacks on the FBI infrastructure but have their own assigned frequencies. I have found them using the 167 - 168 MHz range as well as 170 - 171 MHz but YMMV. One other thing, they are usually P25 and use a NAC code of 293. Best of luck.
True, many of the agencies are on IWN but many of them continue to run with their own radio system. The problem with the IWN systems is that the agents often travel beyond the IWN coverage areas and still need to communicate. When this occurs they have options, operate on their own simplex channels, deploy portable repeaters and or see if there is local radio system they can temporarily slide onto.
Most but not all of the agencies have encryption on all of their radios, however for what ever reason, most of their encryption is not activated all of the time. Some agencies have their encryption strapped with at least one common encryption key that can't be turned off. Encryption incompatibility issues arise when visiting agents are in town with a different encryption key loaded into their radios. OTAR works but not when an agent is beyond the range of an OTAR.
IWN is not without their own problems, the FBI, like other agencies, insist on operating their own systems and on the IWN systems, believe NTIA told them to fund one or the other, last I heard the FBI wants to continue with their own proprietary systems but wanted to absorb the IWN frequencies. Will be interesting to see what happens. The Dept of Justice has the backing and funding to pretty much do what they want.
I think IWN is a great concept and should be mandatory but it should be managed and operated by NTIA.
Wirelessly posted (Opera/9.80 (BREW; Opera Mini/6.0.3/27.2338; U; en) Presto/2.8.119 320X240 LG VN530)
Four years ago, the USMS in Arizona used to be on: 162.7875, 163.200, and 164.600.
These were the reported frequencies used in Tucson, per the Southwest Frequency Directory, 10th Edition.
Today, they are suspected to be on the YRCS (Yuma Regional Communications System) trunked system, TG 310. Their control or dispatch being in the clear, and the deputies being encrypted.