Vonage, the internet telephony group whose shares have fallen 31% since it floated last week, has admitted that it made mistakes in marketing its initial public offering which could give customers who invested the right to seek damages or force the company to buy back stock.

Vonage set aside 15% of shares in its offering for customers in a rare directed share programme. About 10,000 of the firm's 1.6 million customers subscribed to the offer, but were unwilling to buy their allocations after shares fell when they started trading.