With the iPhone 6 I could use the internet and be on the phone at the same time but with the 7 now I can't - did so change ?
Go to settings - cellular - cellular data options
make sure enable LTE is set to voice and data
you must also have an LTE signal. If you display shows 3G or 1x, you can't get voice and data together.
Must be a setting on your device but it is dependent on LTE. Follow what @Weth and if that doesn't work then reset network settings. (or toggle airplane mode)
