(Not saying you’re lying, I just want to learn more!)
For GSM, basic IMSI sniffing: https://github.com/Oros42/IMSI-catcher (usable with cheap SDRs)
For LTE, basic IMSI sniffing: https://github.com/JiaoXianjun/LTE-Cell-Scanner (usable with cheap SDRs)
For LTE, full-on sniffing of all unencrypted metadata: https://github.com/SysSec-KAIST/LTESniffer (generally requires fancy SDRs with GPSDO)
Can vouch that the first and second repos definitely worked as of about a year ago, but haven't personally tried since then. I don't have a USRP X310 (yet!) so can't personally speak to the fancy one.
(Not saying you’re lying, I just want to learn more!)