That certainly explains a problem with SIP if hacking is taking place, which I very much doubt is the situation here.
However none of the big VOIPs use SIP any more, not even 'messenger' which despite my previous post to the contrary, FB now claim they are in fact using full end to end encryption. If they can be believed is for another day, as they say.
I don't think it's a problem with your or your neighbours WiFi. I think it's entirely possible though if Messenger and your VoIP provider use the same insecure protocol (SIP or another).
I'm not an expert in this area but you should be reporting this to EIR or Comreg.
Hi Páid, thanks for your input, yes when I spoke with the Eir Technical dept they did suggest that changing a SIP password was something they would do anyway just in case but they said as we were not on the same network that it was very unlikely to be the reason it happened.
I am considering escalating this matter to make an official complaint as it is some breach of privacy to think you are having a phone call between 2 people but there are possibly others involved and what happened on my line is proof that it happens.