Structure of $p$-adic Schottky groups

For an arbitrary $p$-adic Schottky group $\Gamma$, we construct a set of generators $g_1,\dots,g_n$ with the following property: There exists a set of $2n$ circles $I_1,I_1',\dots,I_n,I_n'$ in the protective line with disjoint interiors, such that $g_i$ maps the exterior of $I_i$ onto the interior of $I_i'$, $i=1,\dots,n$.