Colt Telecom Group said it was for the court to decide who is right in its legal dispute with bondholder Highberry.
Colt has denied claims by bondholder Highberry that it cannot repay its debts and should be put into administration.
Highberry, part of New York-based hedge fund Elliott Group, said it has "legitimate concerns" about Colt Telecom's balance sheet.
"We are getting on with the job of running Colt for the benefit of all its stakeholders. We have already asserted Colt has a sound financial future and that Highberry's claims are self-serving and without merit," said a spokesman acting for Colt.
"There is a legal process that they as bondholders are entitled to follow. It is now for the courts to decide who is right," he said.
Highberry said it is concerned the £1 billion sterling of cash Colt has left, "is being and will be depleted by further expenditure on investments and associated unprofitable trading which are unlikely to produce a reasonable economic return".
Highberry said it believes Colt's assets are "insufficient to meet the total of its liabilities [including its bond liabilities]".
PA