"A peer-to-peer VoIP call occurs when two VoIP phones communicate directly over IP without IP PBXs between them." from msdn.microsoft
My theory is that someone or some robot was dialing directly my IP address, bypassing Voxalot. Since my device had peertopeer_mode set to 1, it allowed that kind of connections. Setting the parameter to 0 seems to have solved the issue. The drawback is that no one can call me directly by IP address, but for me it is not a problem at all. Whoever needs to call me knows my number and gets routed through my voip provider and then through Voxalot, following whatever rules I have configured in both systems.