Apparently this is due to the way that some mobile networks work, you'll hear a dial tone whilst the network is searching for the phone. Dawned if I can find where I read that now though.
The old mechanical exchanges worked by passing the actual pulsed alternating current from the ringing generator through the caller's receiver, the electromechanical switching apparatus in the exchange, the recipient's bell and back to the ringing generator, all in one circuit; so your speaker cone in your receiver moves in and out in perfect sympathy with their clapper striking the two bell gongs in their phone. It could hardly be any other way, when the same pair of wires are used to provide power, audio in both directions and signalling.
This gives you useful feedback, so it was decided to emulate the same behaviour on more complex systems, e,g. where wireless links or analogue to digital conversion are in use (because it's not just one simple circuit with the ringing generator and the two phones anymore; there are now at least two separate "circuits", for audio upstream and downstream, and probably another for signalling).
You can be hearing "ringing out tone" in your receiver; but, despite the strong cultural association, it does not necessarily mean that anything is happening at the other end.
On a digital phone system, each call uses
two separate channels: a "B channel" carrying the zeros and ones that make up the sound, and a "D channel" that is used purely for requesting and receiving information about the status of a call. For instance, when you dial a number on a mobile or VoIP phone, it sends a message down the D channel to the exchange requesting for a call to be set up. The exchange then returns a message down the D channel saying either that it is trying to contact the destination, or that it can't proceed for some reason e.g. their phone is busy, there is no subscriber associated with the dialled number, you are calling anonymously but the recipient does not appreciate the telephonical equivalent of being jumped from behind by a masked stranger, there are no spare radio frequencies to establish a B channel with the base station, you have run out of credit, or whatever; and the caller's phone will generate a ringing-out or busy tone locally. (Anything more complex, such as an announcement that you need to make a top-up and offering the ability to do so now, requires opening a B channel between phone and exchange.) Sometimes, the caller's phone will get no message back from the exchange, but start generating a ringing-out tone anyway while it repeats its request for a status update; and this can cause a sudden switch from ringing-out to busy tone, if the message comes back that the recipient's phone is in use. It is only when the recipient actually answers their phone that a full end-to-end, two-way audio connection is established. Prior to then, everything has just been messages on the D channel. (Of course, if the recipient's phone is a two-wire analogue land line phone, this will still use an alternating current to ring the bell and pairs of tones or series of brief interruptions of the line to indicate dialled digits. The exchange will translate between such "in-band signalling" as it is known, and D channel messages, seamlessly from the point of view of the user.)
tl;dr -- see the bit in
purple.
