According to Giuliani, “If negotiations are not successful and Mr. Trump is subpoenaed, he will fight it.”
From CBS News:
Former New York Mayor Rudy Giuliani, who is now on President Trump’s legal team, told CBS News correspondent Paula Reid Monday afternoon that special counsel Robert Mueller’s office has rejected proposals to allow Mr. Trump to answer questions from investigators in writing.
The president’s legal team has previously signaled that this would be their preferred format for a possible interview as it helps protect Mr. Trump from the possibility of lying or misleading investigators, which is a criminal offense. […]
If negotiations are not successful and Mr. Trump is subpoenaed, he will fight it, Giuliani said. The case would likely end up at the Supreme Court.
Giuliani is not suggesting that Mr. Trump would ignore a subpoena, but rather that they will use it as another opportunity to negotiate an interview on their terms. If that does not work, they will challenge it in court.