\begin{tikzpicture}
\begin{umlseqdiag}
\umlobject[class=A]{a}
\umlcreatecall[class=B]{a}{b}
\begin{umlfragment}[type=alt, label=i>5, inner xsep=5]
\begin{umlcall}[op={call1(i,k)}, dt=7, return=2]{a}{b}
\end{umlcall}
\umlfpart[default]
\begin{umlcall}[op={call2(a,k)}, return=4]{a}{b}
\end{umlcall}
\end{umlfragment}
\end{umlseqdiag}
\end{tikzpicture}