A surprising relationship is established in this paper, between the behaviour modulo a prime $p$ of the number $s_n({\cal G})$ of index $n$ subgroups in a group ${\cal G}$ , and that of the corresponding subgroup numbers for a normal subgroup in ${\cal G}$ with cyclic quotient of $p$ –power order. The proof relies, among other things, on a twisted version due to Philip Hall of Frobenius' theorem concerning the equation $x^m=1$ in finite groups. One of the applications of this result, presented here, concerns the explicit determination modulo $p$ of $s_n({\cal G})$ in the case when ${\cal G}$ is the fundamental group of a tree of groups all of whose vertex groups are cyclic of $p$ –power order. Furthermore, a criterion is established (by a different technique) for the function $S_n({\cal G})$ to be periodic modulo $p$ .