\documentclass[border=20pt]{standalone} 
\usepackage[english]{babel} 
\usepackage{tikz-uml} 

\begin{document} 
\small\begin{tikzpicture} 
  \begin{umlseqdiag} 
    \umlactor{user} 
    \umlobject{AudioService} 
    \umlobject[]{Settings} 
    \begin{umlcall}[op=handleMessage(),return=1,name=a]{user}{AudioService}
        \begin{umlcall}[op=persistVolume(),return=1]{AudioService}{AudioService}
             \begin{umlcall}[op=putIntForUser(),return=1]{AudioService}{Settings}
                \begin{umlcall}[op=putStringForUser(),return=1]{Settings}{Settings}
                \end{umlcall} 
             \end{umlcall} 
        \end{umlcall} 
    \end{umlcall}
  \end{umlseqdiag} 

\node [below,font=\scriptsize] at (a-op.south) {MSG\_PERSIST\_VOLUME};
\end{tikzpicture} 
\end{document}