By Rory Cellan-Jones
Technology correspondent, BBC News
A Spinvox customer claims to have been contacted by a call centre
The voicemail-to-text service Spinvox has applied for two patents which describe the service as being operated by humans, the BBC has learned.
Spinvox has previously claimed that state-of-the-art speech recognition technology is the basis of its service.
However, its patent applications claim the approach is accurate precisely because it employs human operators.
Spinvox's head of social media said that the patents were just two among many for which the firm had applied.
The BBC has also learned that an employee of a call centre in Pakistan tagged his own appeal onto a Spinvox message sent to a US customer, claiming that workers there had not been paid.
Spinvox has always maintained that humans only play a minor role in converting voice messages into text.
Both patent applications were lodged in the United States in the name of Daniel Doulton, the company's co-founder. The first, which was filed originally in 2004, describes a
"method of providing voicemails to a wireless information device"
It says an operator "intelligently transcribes the actual message from the original voice message", and concludes that "because human operators are used instead of machine transcription, voicemails are converted accurately, intelligently, appropriately and succinctly into text messages."
The second application, which appears to be dated July 2nd 2009, describes a similar system,
but goes into more detail
It says "the task of constructing voice recognition software that can reliably and accurately recognise natural speech...remains a daunting one." But it says Spinvox's invention challenges the orthodoxy by using human operators.
When queried about one of these patents by a commentator on the firm's blog, Spinvox's head of social media James Whatley said it was just one among many.
"Spinvox has an entire family of over 70 patents currently residing at international patent offices and many of them have different purposes for their application/submission," he said.
"The one you quote sits firmly under the umbrella of 'the strategic patent'. Generic patents help us build different combinations - i.e.: Humans interacting with machines - to prevent any other companies doing similar things in the long term."
Meanwhile Jason Sewell, a Spinvox customer from Virginia in the US, said he received an email from the service this March that included an audio file of a voicemail message for him.
The text read: "We are employees of Spinvox. Since voicemail to text message service has started by Spinvox we are converting your messages here in Pakistan."
It invited him to ring a number in Pakistan and concluded "You can confirm please we are in real trouble. Please for God sake."
Mr Sewell told BBC News that a friend rang the number on his behalf and was told the workers at the call centre were angry because they had not been paid for two months.
The BBC has asked Spinvox to comment on the patent issue and on Mr Sewell's story.