Hundreds of agency workers have braved freezing temperatures to stage protests across the country against "morally wrong" contracts they claimed were costing some more than £500 a month.
The Communication Workers Union (CWU) said staff were being denied the right to equal pay because of a legal loophole.
Under the Agency Workers Directive, all agency workers should receive the same rates as other workers after 12 weeks in a job, but the union said many were signing away their rights without realising what they were doing.
CWU general secretary Billy Hayes said: "We want agencies and hirers to stop using these contracts, and we want government to close the loophole.
"They may be legal, but they're morally wrong and against the spirit of equal treatment which was at the heart of the new regulations."
Demonstrations were held outside call centres in areas including Blackburn, Canterbury, Doncaster, Lancaster, Lincoln, Nottingham, Dundee and Glasgow.
CWU official Roy Reynolds joined a group outside a BT call centre in Doncaster, saying the response to the protest had been "massive".
BT said in a statement: "We negotiate pay levels at a national level with our recognised trade unions and we are very confident that we have appropriate pay levels for both our employees and the business.
"In 2012, our investment plans created around 2,000 permanent jobs, including the opening of four new UK call centres and recruitment of engineers to support our fibre broadband rollout.
"Many agency staff took the opportunity to move into these permanent new roles at the time."