The 21-year-old explained that he was at home when he took the call with news of the development that would change the course of his career and offer him a route back into Premiership football.
It was a whirlwind 24 hours for the striker, who said it is difficult to explain the gamut of emotions that come with a big move - particularly one that happens on the brink of a window deadline.
"It is hard to put into words really because I was just sitting there on Sunday night and got the call," he revealed. "I couldn't believe it, I thought 'cor, this is last minute' and you always think the worst worrying it won't go through.
"I really wanted it to happen and I am delighted.
"I think it was about ten to five on Monday when I actually signed so it was a bit last minute. I'm glad that it's all sorted out and here I am.
"It is a good opportunity for me to be back in the Premiership, it is where I want to be and I am looking forward to the challenge."
With Bobby Zamora switching to Upton Park, both Hammers boss Alan Pardew and David Pleat stated that both sides are happy with the deal - a fact that pleases Jermain, who makes no secret of his fondness for his former club.
"Definitely, I love everyone there and the fans have been great to me. I hope they do well, they have a young side there now and have a good chance. It is a good deal for both clubs.
"I am just looking forward to it here now, I want to get out there and get on with it."
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Jermain Defoe admitted that his only doubt when the surprise call came on Sunday evening to tell him that a transfer had been agreed between West Ham and Spurs was that the deal would not be concluded on time.