What are the dangers associated with getting a call from a random number?

If you get a call from random number -

I wondered if the phone call to my mobile could have stayed connected after the phone call 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 activity, you'll get notified. For exploiting or requesting any of your information, he needs 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 your data through a phone call without any repercussions because of the secured VoIPs and Tele-Exchanges.