Vontoo's technology allows companies to send and track marketing voicemail messages targeted to a particular audience. The system can also be used to send notifications and alerts, and integrate voice communications into a company's existing software applications.

Among Vontoo's highest-profile clients to date have been sports franchises such as the Cleveland Browns and Memphis Grizzlies, which have used its voice-messaging services to drive ticket sales.

The company recently said its voice-marketing tools comply with a new Federal Trade Commission rule requiring telemarketers to provide consumers with an opt-out mechanism such as a key press at the start of all automated messages.

Vontoo said its system was designed to allow someone to opt out at any time during a call. But the FTC rule--effective since Dec. 1-- specifically requires the opt-out message to play at the start of sales and solicitation calls, and that messages left on voicemail systems or answering machines provide a toll-free phone number to prevent further calls.

Roughly three-quarters of the company's latest investment round came from existing investors. New investors include Gregg Throgmartin, a shareholder in appliance retailer H.H. Gregg. His investment followed the company's use of Vontoo's services. Vontoo initially raised $1.8 million a year ago.

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