People who have left it until the last minute to renew their tax credits have been urged to "keep trying".
Saturday is the final deadline for renewal and extra staff were brought in to answer calls to HM Revenue and Customs (HMRC), a spokesman said.
Some people struggled to get through because of the sheer number of calls but an HMRC spokesman urged people to "keep trying".
"People have got to renew today," he said.
"We have brought in extra people to try to answer the phones.
"Historically, it's always busy around this time.
"All hands are at the pump."
Film-maker Tricia Emery, 39, said she had tried the HMRC phoneline around 30 times on Friday and Saturday with no success.
Ms Emery, 39, from London, said there was no option to wait on the line and she kept being cut off because nobody was available to take her call.
"They should have a waiting system where you are in a queue, even if it's for an hour."
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