Carlyle Capital's phone is ringing with margin calls, but the company isn't picking up.

The publicly traded mortgage fund managed by the private-equity powerhouse Carlyle Group, whose founders include David Rubenstein, has received one notice of default stemming from four missed margin calls, and it expects to receive at least one more.

It said it received seven margin calls yesterday totaling $37 million, and it was able to meet requirements for three of them. The fund's $22 billion portfolio uses short-term funding agreements with banks, known as repos, to fund its investments in certain Fannie Mae and Freddie Mac bonds, which are rated AAA. The repurchase agreements have an average outstanding maturity of twenty days.

The company said the repo counterparties' margin prices are not representative of the underlying value of the securities. "Unfortunately, this disconnect has created instability and variability in our repo financing arrangements," said Carlyle Capital chief executive John Stomber in a statement. "Management is actively working with the company's repo counterparties to develop more stable financing terms."

It said it was able to meet the three margin requirements because the counterparties that issued them "have indicated a willingness to work with the company during these tumultuous times."

But clearly the banks behind the loans are concerned. "The banks are tightening credit standards and trying to curtail the lending they do," Citigroup credit strategist Hans Peter Lorenzen told Bloomberg. "One of the ways you do that it is increase haircuts you charge."

Carlyle said it has received margin calls totaling $60 million in the past week.

This is just the latest in a string of problems that the company has faced since it went public last summer. Just a few weeks after shares were initially listed at $19 on the Euronext Amsterdam exchange, it was forced to borrow $200 million from the Carlyle Group in order to meet margin calls on leveraged loans.

Last week, the company pledged to do better after it announced that it lost 30 percent of net assets in the second half of last year.

The shares are down 2 percent in Amsterdam today, to $11.80.