Date: 202107
Local time of day: 0601-1200
Location: SPI.TRACON, IL
Aircraft: Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Operating under: Part 91
Flight plan: IFR
Flight phase: Cruise
Airspace: Class B STL
ATC / advisory: TRACON SPI
Second aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Procedure
Primary problem: Procedure

Narrative: While working the radar position; I accepted an automated handoff on Aircraft X; an IFR flight inbound from the St. Louis area to one of our satellite airports. Several minutes later I received a transmission from Aircraft Y. Because I was not expecting to hear from this aircraft--looking back; it appears they mistakenly took the frequency change meant for Aircraft X--I queried their position and they said they were in the vicinity of the STL VOR. I advised them that they were well outside my airspace and suggested they contact St. Louis Approach (T75) on the 'Low North' position frequency 124.2. I then realized that; if they were close to the STL VOR; they probably were not looking for initial services but were already in contact with ATC. I immediately reached out again; asking if they had been talking to a T75 Controller before contacting me; but they had already changed away from my frequency. I reached out to the Low North Controller on the landline and told them what had happened; and that the aircraft was no longer responding to me. They said they were trying to get in contact with the aircraft and there may have been an NMAC over the airport. Eventually they told me they had re-established contact with the aircraft. Additionally; while this was happening; Aircraft X flew beyond radio coverage of the T75 Low North position without contacting me. Eventually they reached out on my frequency and I advised the Low North Controller that the aircraft was no longer NORDO. That was the end of it until; while going home; I thought over the situation again and realized I wasn't certain if I had suggested that Aircraft Y contact St. Louis on 124.2 or 124.3. 124.2 is the correct frequency for the T75 Low North position; while 124.3 is the frequency for ZKC Sector 58; which borders the east/southeast side of both our airspace and T75's. I can't remember which frequency I suggested; if I did suggest the wrong one that would have contributed to the confusion and extended the amount of time the T75 Controller was unable to properly control the aircraft.  If I had kept the pilot on my frequency while I reached out to the T75 Low North controller; I would have been able to issue a confident 'contact' instruction instead of 'suggest you contact.' If I had issued that instruction I would have been less likely to make a slip and say the wrong frequency. It is also surprising to me that the frequencies 124.2 and 124.3 are used in immediately adjacent sectors. This is a potential cause of confusion for controllers and pilots alike.




Synopsis: SPI TRACON Controller reported an aircraft calling on wrong frequency that was not under their control which resulted in a NMAC.