The Prime Minister also met with Labour MPs in a bid to gain support for her proposals ahead of next Tuesday’s Commons vote.
Mrs May held phone conversations with Unite boss and Jeremy Corbyn confidant Len McCluskey as well as with Tim Roache of the GMB.
Number 10 described the calls as “constructive” and confirmed ministers would "consider very seriously" moves by Labour MPs to safeguard workers' rights after Brexit.
But following the calls on Thursday, Mr Roache said the non-binding assurances on workers’ rights in Mrs May’s deal current deal “won’t cut it”.
Mr Roache added: "After nearly three years I'm glad the Prime Minister finally picked up the phone.
"As you would expect, I was very clear about GMB's position, the deal on the table isn't good enough and non-binding assurances on workers' rights won't cut it.
"If the deal genuinely did the job for GMB members, our union would support it, but it doesn't."
Shadow foreign secretary Emily Thornberry later on Thursday hit out at Tory figures putting out feelers on workers' rights in a bid to get a Brexit consensus.
Appearing on the BBC's Question Time, she accused the Government of treating the issue as an "afterthought" by attempting to discuss it at such a late stage.
She said: "It would be fundamental to what we were doing and we would have it as part of the law that our rights would be in pace with Europe.
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"For the Tories to come back at the last minute and go 'oh yeah you know that thing about workers' rights, well you might be able to have the same rights as they have in Spain', it just shows the fundamental difference of approach to this."
Mr Corbyn said the widely-expected defeat for Mrs May's deal next Tuesday would signal the failure of her leadership and of the Conservatives as a party of Government.
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Speaking on a visit to Wakefield in Yorkshire he urged MPs from across to House to back the motion of no confidence in the Government which Labour would table "at the moment we judge it to have the best chance of success".
Asked if he agreed with shadow Brexit secretary Sir Keir Starmer that an extension to the two-year Article 50 process may now be "inevitable", Mr Corbyn said: "An extension would be a possibility because clearly there would have to be time to negotiate."