Not to be confused with ringtones -- which many people are now comfortable with buying over-the-air, sometimes for polyphonic handsets -- ringback tones involve the calling party hearing a song or film excerpt or DIY message left by the called party instead of a brrring brrring sound (or brrrrrrrring in some countries).
This is a further phase in mobile phone personalisation and one that analyst house Ovum today said will be worth $721m (£401m) in Western Europe in 2008, up from around $16m this year.
However, in only February this year, mobile entertainment company Netsise put a figure of $1.5bn across Europe by the end of 2005.
Such forecasts were somewhat understandable. In South Korea, a spin-off from SK Telecom called widerthan.com has done well, enabling SKT's ColoRing service. It costs consumers around $2 per month, which now translates into $8m in monthly revenue and 30 per cent user-base penetration.
Ovum points out that in the Philippines, the launch of a service from Global Telecom led to 100,000 eager users in its first week.
But all is not well. Ovum and others say ringback tones are difficult to explain and therefore market and managing the service can be complicated at a network level for operators, at device level and in terms of securing rights with content companies such as music labels.
T-Mobile has had some success in Europe since its ringback launch across the Czech Republic, Germany and the UK at the end of last year, with other operators such as Tele2 Sweden, Telefonica Spain and Vodafone Germany now following. But all may want to note the performance of operators such as M1 in Singapore and NTT DoCoMo who haven't done well - and that's in Asia where such services are generally thought to catch on faster.
Ovum said Western Europe is set to account for around 30 per cent of ringback tone sales.