This year, 5.3 billion mobile phones will be thrown away not recycled, the international waste electrical and electronic equipment (WEEE) forum says.
Its estimate, based on global trade data, highlights the growing environmental problem of "e-waste".
Many people keep old phones, rather than recycling them, research suggests.
Precious minerals not extracted from waste electronics, such as the copper in wire or the cobalt in rechargeable batteries, have to be mined.
"People tend not to realise that all these seemingly insignificant items have a lot of value and together at a global level represent massive volumes," WEEE director general Pascal Leroy said.
There are an estimated 16 billion mobile phones worldwide - and in Europe, almost a third are no longer in use.
The WEEE says its research shows the "mountain" of electrical and electronic waste - from washing machines and toasters to tablet computers and global positioning system (GPS) devices - will be growing by 74 million tonnes a year by 2030.
Earlier this year, the Royal Society of Chemistry launched a campaign promoting the mining of e-waste to produce new products, highlighting global conflict, including the war in Ukraine, threatens precious-metal supply chains.
Magdalena Charytanowicz, of the WEEE, said: "These devices offer many important resources that can be used in the production of new electronic devices or other equipment, such as wind turbines, electric car batteries or solar panels - all crucial for the green, digital transition to low-carbon societies."
Just over 17 per cent of the world's e-waste is properly recycled - but the United Nations Internation Telecommonucation Union has set a target to raise that to 30 per cent by next year.
It highlights it is one of the "fastest growing and most complex waste streams that affects both human health and the environment, as it can contain harmful substances".
In the UK, more than million unused but working electrical items, worth as much as possible £5.63 billion, are currently hoarded in UK homes, surveys by the organisation Material Focus suggest.
It also calculated that the average UK household could sell unwanted tech and raise about £200.
The organisation's online campaign provides tips, including where to find recycling centres.
Mr Leroy said much more could be done.
"Providing collection boxes in supermarkets, pick-up of small broken appliances upon delivery of new ones and offering PO [post-office] boxes to return small e-waste are just some of the initiatives introduced to encourage the return of these items," he said.
