T-Mobile/Wifi-calling: Data usage on home wifi counts as mobile data

Recently, I switched to T-Mobile USA with my iPhone 5S and enabled WIFI-calling. Since then, my mobile data volume has gone up drastically. T-Mobile counts data usage on my home wifi network as well to the mobile data volume. I know this, because there is mobile data traffic stated on my bill during times when I was at home for sure, i.e. at night. T-Mobile says they can't do anything about it and I should just buy more full speed data volume. Does anyone have the same issue?