Distributed voltage control using Q(U)-characteristics is a means to facilitate higher penetrations of decentralized generation. However, stability concerns interfere the wider adoption of this technology. Motivated by a research project on ancillary services in electrical subtransmission and distribution networks a verification of limits in grid codes was carried out. This paper analyzes the stability of a network with distributed voltage control, which is modeled as an abstract nonlinear multiple input, multiple output system. Applying theoretical stability criterions show that the limits of stability can safely be derived for known parameters of the controlled units. For the more praxis-orientated case without detailed knowledge of the controlled units, a robust small-gain-based stability criterion is presented. Based on this, easy to use rules to ensure the stability of decentralized voltage-controls are derived and verified using simulations in a benchmark grid.