Using a \phantom{} in \ul from the soul package

Question

In responding to how to typeset a Long underscore in LaTeX, I attempted to use a \phantom with \ul, but end up Argument of \makeph@nt has an extra }. So, what is the proper way to use a \phantom command in such a situation.

\documentclass{article}
\usepackage{xcolor}%
\usepackage{soul}%

\newcommand{\UnderlineText}[2][red]{\setulcolor{#1}\ul{#2}}%

\begin{document}
    Signature: \UnderlineText[blue]{Sign here}

    %Signature: \UnderlineText[blue]{\phantom{Sign here}}% Error if un-commented.
\end{document}

Answer 1

If you wrap the \phantom{...} in an extra set of curly braces, the code works:

\documentclass{article}
\usepackage{xcolor}%
\usepackage{soul}%

\newcommand{\UnderlineText}[2][red]{\setulcolor{#1}\ul{#2}}%

\begin{document}
    Signature: \UnderlineText[blue]{{\phantom{Sign here}}}
\end{document}