Danger of making calls after unknown random number called through

Question

I recently received a call from a number I didn't know to my mobile, answered it anyway. I didn't say anything, when it is a number I don't know I let the other person speak first.. after a few seconds line went down, tried to call back but phone just made an odd noise instead of ringing and it was similar to a fax but only for around a few seconds and then went dead. I wondered if the phone call to my mobile could have stayed connected for after the phonecall ended in any way? I believe in this scenario the unknown caller ended the call, but my question is regarding either party ending the call and the maximum time a phoneline can stay open for ears dropping any next call?

I made a call around 45 minutes after about something unrelated and feel concerned if there could be any personal information leaked in that call because of if any line is still connected from the unknown caller.

Answer 1

It depends on the PSP's (Phone Service Provider's) end on how their systems are configured. Basically the disconnect happens between the phone on the other end and your phone, only after the system on your PSP's end is somehow informed of a hangup on the other end, on your end, or is able to detect it, does it start disconnecting the line.

The amount of time from the start of line disconnection to the actual disconnection depends on how busy the system is to perform the disconnection, and where the hangup originated from.

When the other person hangs up and you still hold on, you probably will hear two noises (after the receiver's phone hangs up). The first is the disconnection on the other ends PSP's and the next is on your ends PSP's. This can more easily be heard on older copper lines.

As per the wierd sound you heard, it is not common that small phone exchange systems can be hacked [1]. The hacked number may be that of a fax machine.

Answer 2

I wondered if the phone call to my mobile could have stayed connected for after the phonecall ended in any way?

No, if the phone call has been disconnected, the PSP's(Phone Service Provider's) switch is rotated to another communication channel. And most of the SPs are not so poorly configured that they won't show out the call status disconnected even if the call's connected.

If you're using a residential land-line, you'll hear a hopping frequency after the call disconnects. The fax voice which you heard is probably the voice of the communication switch.

I feel concerned if there could be any personal information leaked in that call.

No call can deliver any type of payload to any device. Even if the PSP is breached, still the call switching can drop any incoming payload before reaching the destination header as the hopping packets are different.

If talking about the worst case scenario, let's say he made it out with his payload to your device. If your device is Android>=6, for every installation/malicious activities, you'll get notified. For exploiting or requesting any of your information, he need to have privileges to perform the action.

CVE-2019-18683 can be used to escalate the privileges through race conditions of a lower-end user to have permissions to gain access to data without getting privileged. But that action gets performed at a delay of the payload delivery to any system. You can get alarmed before anything happens and can stop that as the system will detect which service needs the privileges.

Plus, this vulnerability was only for Android 6 M devices and Linux Kernels 5.3.8, later versions have patched this vulnerability.

However, in any case, the attacker cannot steal out your data through a phone call in any repercussions because of the secured VoIPs and Tele-Exchanges.