In theory, it turns the medium into an instantaneous forum for debate.
What that means in practice, though, is the dark misanthropy of maudlin cabbies, attracted in droves to phone-ins in the wee small hours, and the mentally unstable football supporters who confront Richard Littlejohn on BBC Radio Five Live on Saturday evenings.
But the telephone-radio relationship is changing, and you don't have to be a rocket scientist, or even a poptastic DJ, to work out why. As in other media, the driver is digital, convergent technologies, and they're having an impact in two distinct but related ways.
The first is all about hardware and the fact that mobile phones aren't just phones any more. They are many things these days, from PCs and personal organisers to games consoles - but most importantly, they're radios too.
The latest generation of mobiles have analogue radio decoder chips as a standard part of the kit. The next generation will have digital decoders.
This is exciting for media owners because there could be a windfall in terms of increased audience figures.
In fact, Justin Sampson, the managing director of the Radio Advertising Bureau, argues, this could be another landmark moment for the medium.
The mobile phone, he believes, could be to the Noughties what the transistor radio was to the 60s: "It could help radio to reinvent itself. People always have their phone with them, so listening will increase because of that. Listening has already been increasing at home in the evening, and at work people are listening through the internet. This could be yet another front on which radio could grow its audience."
This will obviously please advertisers. But they'll be even more pleased by windfall factor number two - the medium's increasing tie-in to the biggest craze of the new millennium - SMS text messaging. The equation is simple. If you run an ad campaign or a promotion that invites an SMS response, the response will probably be highest among those who are actually listening via their mobiles. In the future, there might be very simple "push one key to respond
But the ultimate prize is even more sophisticated than that. It involves the creation and astute use of permission-based marketing databases.
You could ask listeners to respond to a promotion via SMS and, while you're at it, ask for some personal details and if they would mind you contacting them later about other products or promotions. Or you could form an SMS club offering games and gossipy text messages, plus promotional station material. Once you've built a close relationship with these people, you can invite advertisers to be involved with future promotions.
For example, a radio station could offer film tickets as a competition prize in association with a film distributor. You could even ask members of the "club
if they want to receive straight advertising over their phones. Many do.
Is this sort of initiative going to be of increasing interest to advertisers?
Is it about to take off in a big way? Jonathan Gillespie, the head of radio at OMD, is slightly sceptical. "Radio welcomes any type of back-channel capability and agencies are certainly looking to test the waters more, especially for advertisers whose target market is younger. Media owners have to set out their stall and prove their willingness to take the game on. But you have to have some common sense. Radio has great strengths as a medium and you shouldn't be looking to turn it into something else,
Robert Horler, the managing director of Carat Interactive, agrees that we shouldn't get carried away just yet: "You have to look at some of the costs to the consumer, which remain high. What does it say about you as a brand when you ask people to pay to find out that they're losers?
"It's a bit like viral marketing: a few of the creative ideas are great but most are wishy-washy. There's a fine line between being quirky and clever and doing something that is tedious."
He also argues that the redemption mechanisms will have to become slicker: "Couponing is something you can do only on a limited basis because, at the moment, you have to hold your phone up to the shop assistant and it's open to fraud. It won't really work until your phone essentially becomes an electronic wallet that can be scanned.
"Until that happens, it will be difficult for major companies to do nationwide promotions. It's impossible to manage. But there is potential and there are lots of ideas around. Radio owners understand the potential more than most."
According to mobile marketing specialists, media owners also understand the pitfalls more than most - and this is definitely an area that can become counter-productive if it isn't handled with sensitivity and finesse.
Pamir Gelenbe, the director of corporate development at flytxt, says it is absolutely essential to respect the basic rules of permission-based marketing. "Don't abuse the data you have. Maintain the relationship. Keep interest alive with innovative content. Offer an easy opt-out channel; if they haven't opted out, you can therefore assume that the interest is still there,
The need to keep campaigns fresh and lively is something the media owners are well aware of. And the other issue on which everyone tends to agree is that this market will evolve rapidly, especially with even more impressive internet-enabled mobile technology on the horizon.
Ian James, the interactive sales director of Chrysalis Radio, reckons the media owners have been getting it pretty much right so far and, in a way, there's really no excuse for getting it wrong, seeing as audience interactivity is the name of the game.
He concludes: "We have pushed on in a cautious way. Text messaging increases listener loyalty, gives added value to promotions and can be used as a standalone advertising medium using an opt-in database.
"And we know all too well that there are a lot of cowboys out there in the mobile market who don't respect the privacy of the people on the database. You have to make sure that every message has value to the end user. You have to let it evolve and you really do have to listen to what listeners want."