BT's problems with its 1571 answering service have been tracked down to a computer server in Brighton.
Customers contacted the company after they found the free service was not working.
BT say weather conditions could be one reason for the breakdown.
A spokesman said: "There is a problem with one of our computer servers that is one of 13 covering the country.
"At this stage I don't know how many customers are affected or the cause, although we are aware of the problem and are in the process of fixing it."
Although the computer server, or "platform" as it is known, is based in Brighton, customers elsewhere in the country could be affected.
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