The X Factor boss is said to have told the One Direction singer to stop ignoring Swift's calls, warning that doing otherwise could harm the band's US success.
While the former couple aren't expected to get back together following their split, they have now cleared the air, according to the Sunday Mirror.
"Harry still really likes Taylor but, like anyone who has gone through a break-up, he's found it difficult to speak to his ex," a source said.
"After flying back to the UK, Harry's been trying to figure things out and in order to do that he decided to initially avoid her calls.
"Simon gave Harry a friendly warning that any animosity between him and Taylor could jeopardise One Direction's popularity in the US.
"They're two of the world's biggest music stars, and Simon knew that, although things didn't work out between them, they were bound to meet at various events and awards ceremonies.
"Harry spoke to Taylor on the phone while on a trip to Ghana with the One Direction boys this week for Comic Relief.
[One Direction in Ghana]
"Taylor was keen to meet face to face and talk things through but Harry felt there was nothing left to be said. Plus the boys' schedule meant it wouldn't have been possible as they were due to fly to Japan.
"Harry told Taylor he didn't hold grudges and hoped things wouldn't be awkward when they saw each other. Taylor seemed to listen but insisted there were still things they needed to discuss.
"The air has been cleared between them and they're due to see each other at next month's Brit Awards, although friends think Taylor may pull out."
A spokesperson for Simon Cowell has denied the Sunday Mirror's claims, commenting: "This story was completely made up, One Direction make their own decisions on their personal relationships and this matter hasn't even been discussed. There's absolutely no truth in this."
Swift is said to have written five songs about Styles since their short-lived romance ended.
Photo gallery - Harry Styles's rumoured girlfriends: