We report on the first observation of a carrier-concentration-induced phase transition in (Pb)SnMnTe, a system dominated by RKKY exchange. The magnetic properties of Pb1-x-ySnyMnxTe, 0.01<x<0.03, 0.72<y<1-x were studied in magnetisation, susceptibility and specific heat experiments. We shall show that for carrier concentrations, p, between 3×1020 and 1021 cm-3 a ferromagnetic state occurs, while for p above 1021 cm-3 the system turns into a spin-glass-like state. This phase transition can be identified with the transition speculated about in RKKY-driven systems. The phase diagram in x-p parameter space will be derived using a simple geometrical argument, and in terms of a Sherrington-Kirkpatrick model calculation for spin glasses.