Orbot in vpn mode should prevent websits from knowing your original IP but IP is not the only way your phone can be identified. Its possible you need to check your settings to see what else could be leaking info.

More accurately Orbot is a proxy not a vpn, but on mobile devices its usually easier to just use the "vpn mode" so traffic is always routed over tor instead of having to manually set proxy for each app you want to use tor with.