escape underscores

Set a \rule of specified width:

\documentclass{article}

\newcommand{\format}[2][-.2\baselineskip]{\rule[#1]{#2}{.4pt}}

\begin{document}

Format (\format{2em}) \format{2em}-\format{2em}-\format{2em}-\format{2em}

\end{document}

Some vertical adjustment can be tweaked by trying different lengths. The optional argument to \format sets the rule .2\baselineskip below the baseline by default.

---

\documentclass{article}

\newcommand{\format}[2][2pt]{\underline{\hspace{#1}\phantom{#2}\hspace{#1}}}

\begin{document}

Format (\format{000}) \format{000}-\format{000}-\format{000}-\format{000}

\end{document}

The above option uses an \underlined \phantom with some padding.

I suggest making \format to use a sample argument to define the width (plus buffer, in this case 2pt). For example, \format{000} will make the underline the width of "000" plus 2pt.

\documentclass{article}

\newcommand{\format}[1]{\setbox0=\hbox{#1}\rule[-.2\baselineskip]{\dimexpr2pt+\wd0}{.4pt}}

\begin{document}

Format (\format{00}) \format{000}-\format{000}-\format{000}-\format{000}

\end{document}

If one wanted to account for the difference between typeset versus handwritten lettering, one could apply a multiplier to \wd0, as in

\documentclass{article}

\newcommand{\format}[1]{\setbox0=\hbox{#1}\rule[-.2\baselineskip]{\dimexpr3pt+1.5\wd0}{.4pt}}

\begin{document}

Format (\format{00}) \format{000}-\format{000}-\format{000}-\format{000}

\end{document}