If your iPhone is old enough that the jack on it accepts a stereo mini jack rather than the 'Lightning' plug of new phones the signal coming from it is analog, not digital, so it can't be plugged into a digital input on the Cambridge. Here's some general information.
https://www.musicservertips.com/how-to-articles/iphone-digital-audio-output/#:~:text=With%20the%20advent%20of%20iOS,audio%20converter)%20with%20USB%20input.
One way to go is to transmit Bluetooth from the phone to a Bluetooth receiver that can be plugged into one of the digital inputs. Here's one with optical output.
https://www.monoprice.com/product?p_id=38071
The new Audioengine B-Fi would let you use WiFi for the streaming rather than Bluetooth but I can't say whether the additional cost would be worth it considering the resolution of Spotify files.
https://audioengineusa.com/shop/adapters/b-fi-multiroom-music-streamer/
https://www.musicservertips.com/how-to-articles/iphone-digital-audio-output/#:~:text=With%20the%20advent%20of%20iOS,audio%20converter)%20with%20USB%20input.
One way to go is to transmit Bluetooth from the phone to a Bluetooth receiver that can be plugged into one of the digital inputs. Here's one with optical output.
https://www.monoprice.com/product?p_id=38071
The new Audioengine B-Fi would let you use WiFi for the streaming rather than Bluetooth but I can't say whether the additional cost would be worth it considering the resolution of Spotify files.
https://audioengineusa.com/shop/adapters/b-fi-multiroom-music-streamer/