Skype has informed some customers that the London telephone number they use for incoming calls will stop working next month.

Some numbers starting with "0207," the prefix for central London, will stop working on 20 December, Skype wrote in an email to customers.

"We're very sorry to tell you that we have to change your SkypeIn number," the email said. "As some of you may know, we get SkypeIn numbers from a variety of telecoms suppliers. Unfortunately, we have to return some of the 0207 SkypeIn numbers to one of our suppliers of London numbers."

Skype sells SkypeIn numbers which let subscribers receive calls on their computer from someone calling from a regular mobile or landline phone.

The problem points to a commercial dispute between Skype and its supplier. Service providers such as Skype buy banks of numbers from telecoms companies, who request numbers in batches of 10,000 from the UK telecoms regulator, the Office of Communications (Ofcom). The commercial agreements between service providers and the telecoms are not regulated by Ofcom.

The central London Skype numbers affected, in the range 0207870 and 0207871, are managed by Gamma Telecom Holdings, a UK company specializing in voice over IP applications. Gamma could not be reached for comment.

Other converged communications companies combining telephony services with an online component have also run into difficulties in providing perpetual phone numbers. Google had to change 400 phone numbers after its July acquisition of GrandCentral Communications, a call-management company, when a supplier stopped connecting the calls. GrandCentral had promised a phone number "for life."

Skype is offering those affected by this latest change a new SkypeIn number for free with voicemail for 12 months.

Due to a shortage two years ago of numbers with prefixes "0207" and "0208," Ofcom started issuing "0203" numbers to telecommunications operators. Skype said the chance of getting another "0207" number is low, but that "0208" and "0203" numbers will be available.

That may be of little consolation to users, one of whom wrote they "just had to cough up $30 for new business cards" on Skype's forum.