\begin{tikzpicture}
\begin{umlseqdiag}
\umlobject[class=A]{a}
\umlobject[class=B]{b}
\begin{umlcall}[op={call(i,k)}, return=2]{a}{b}
\end{umlcall}
\end{umlseqdiag}
\end{tikzpicture}