A team of researchers from French company P1 Security has detailed a long list of issues with the 4G VoLTE telephony, a protocol that has become quite popular all over the world in recent years and is currently in use in the US, Asia, and most European countries.
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Researchers say that an attacker on the same network can send modified SIP INVITE messages to brute-force the mobile provider and get a list of all users on its network.
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This could be an issue with lawful interception (surveillance) because it allows possible crime suspects a way to create covert data communications channels.
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Researchers warn that this is a “critical” issue that may result in attackers accessing another person’s voice mail, or could cause problems for law enforcement monitoring criminals, who would be able to avoid surveillance by placing calls from another phone number.

Not mentioned by researchers, but a plausible scenario, is if tech support scammers would spoof the phone numbers of legitimate companies to call customers and obtain sensitive information such as passwords, card PINs, and other.
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Researchers recommend that mobile telcos sanitize the headers of “200 OK” messages and remove any equipment info that may allow an attacker to create a virtual map of its network. This information is dangerous because it allows threat actors to plan and carry out finely-tuned attacks against the mobile operator.
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Researchers discovered that by watching VoLTE traffic on an Android that’s initiating a call, intermediary messages exchanged before establishing a connection reveal information about the callee (victim)’s IMEI number.
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attackers could initiate shadow calls, detect the victim’s approximate location, and hang up before the phone call is established.

Source: Hackers Can Spoof Phone Numbers, Track Users via 4G VoLTE Mobile Technology