Your internet service provider or mobile carrier can see that you're connected to a VPN, but it won’t be able to see the websites you visit or browsing history, because your iPhone VPN encrypts your online traffic. Your ISP/mobile carrier will also see things like the IP address of the VPN server you're connected to, the timestamps of when you're connected and the amount of data you've transmitted. If you want to hide the fact that you’re using a VPN altogether, you can connect to an obfuscated server, if offered by your VPN.