Frei claims this is the third time Celtic have shown interest in him
Switzerland and Borussia Dortmund forward Alexander Frei says Celtic are keen on securing his signature from the Bundesliga club.
The 28-year-old also said that Celtic previously showed interest in him in 2004 and 2006 and indicated that he is willing to talk to the Glasgow side.
"No-one has ever put an offer on the table," he said.
"If they want me they can phone me. Otherwise, I don't want to talk about it," he said.
Swiss captain Frei saw his tournament come to an early end at Euro 2008 after suffering a knee injury in his side's opening fixture against the Czech Republic.
His recovery is expected to take six weeks.