At a morning, I tried to open mobile data for jami on my smartphone (android 5.0.2). I tried to send two small messages ("test") for two people who was not connected. After few hours (3 to 6h), it uses about 17Mo of datas, and at the evening of the same days (12 to 15h), I saw that it uses more than 42Mo of datas… and I have a small phone package with only 50Mo/month. I use DAVx to synchronize my NextCloud agenda and it is less than 2Mo consumption in a month! I suppose the issue is because receivers can't be reach, and jami try very regularly to send my messages?
Even if lots of people have a non limited data uses in their phone package, I think it is not great to use so much datas!