I have had a 17 year old foster-child bug our house, it is such a feeling of breech of trust, it made me feel quite paranoid for a long time after. When it first started I was not sure whether it was my phone or a bug in the house. We did a "sting" to find out and eventually to stop it. We never found the actual device or the child would have been taken from our house very quickly.
I realised it quite quickly as it was obvious that our conversations were being overheard. My dh took longer to convince, it took me pointing out several "coincidences" for him to start believing me. Iniitally I was convinced it was my phone, (it was losing battery getting hot etc but it was an old phone, so I was not sure). I realised that it was happening in a particular room (the room next to our bedroom that we used as a bedroom). We had already planned to make it into our youngest son's bedroom, so we talked about it in that room.
Initially it appeared to have stopped the listening in, and we went out to the garage or car to have any private conversations. Then we started to feel that maybe it had been all in our heads etc and tried to behave a bit more normally. However then conversations we had in our bedroom we clearly being picked up.... at that point one evening when out with friends we told them of my paranoia ! We laughed about it and planned a "sting" we decided to talk about a topic that we knew the fosterchild would be totally unable to ignore. That night in bed my dh and I in a joking way talked about it, lots of nodding and winking at each other and grinning about it. We had decided that it was just me being paranoid and were laughing it off. BUT it backfired on us, the following morning our foster-child brought up the very topic we had discussed, out of the blue, totally unnaturally. It was then we realised that it was not a joke.
How we dealt with it was for my husband that night to actually say to me clearly that if we ever found a bugging device in our house that our foster-child would be gone immediately, that there would be no second chances and we would seriously consider taking it to the police. It worked, we never had any further issues. Whatever device was there was removed, and for the rest of the time the child was with us, a video recording was set up in our bedroom any time we were out of the house.
It took me a long time to feel the paranoia leave me.
OP - in your situation, I can see that when you move that you are careful with checking out your new house. Right now I would change your phone if at all possible or perform a "sting" with your phone outside of the house to see if your phone is being listened into. I can't imagine it will be easy to cut out the grandparents from your children's lives, but it might be needed for you all. I would advise that you don't let it just be left but do get a good check of your house. Since you say the common factors for the two phonecalls to your sister is your phone, her phone and the sitting room, then those are the best places to start checking.