Can you control the volume on the tv with your phone for apple tv

That is only supported for setups with an AV receiver that supports volume control over CEC, and also for select AirPlay/​Bluetooth speakers (e.g. headphones). Not all devices with CEC support include functionality for volume control from other devices. TVs typically do not support receiving volume control signals over CEC.

The Siri Remote uses IR to send volume control signals directly to the TV or audio system, in all other situations. Mobile devices do not have an IR transmitter, so the software remote wouldn’t be able to emulate this.