About 7bn euros worth of marks are said to still be in private hands
Germans still hoarding old deutschmark coins can now put the former currency to good use - half-price calls at 50,000 public telephones.
Deutsche Telekom is to accept mark coins at a rate of one to one euro at its telephone booths across the country until the end of August.
Germany's central bank estimates that 7bn euros-worth ($8.6bn; £5bn) of marks are still in private hands.
The country's shops occasionally run promotions allowing the use of marks.
Deutsche Telekom's promotion will allow people to use old 10 and 50 pfennig pieces, in addition to one, two and five-mark coins.
Approximately 50,000 public phones will accept the marks
The much-prized old currency was officially removed from circulation to make way for the euro in early January 2002.
Half the value of today's euro, Deutsche Telekom's offer effectively gives users half-price phone calls.
Some analysts said the promotion is designed to publicise the company's public phone boxes, which have seen usage plummet since the growing popularity of mobile phones.
"We hope that we can persuade people to bring out their old coins," a spokesman said.
"Coin telephones are still useful in this age of mobile phones."