It’s not possible to control the Amp via cables, but that’s not necessary.  You could get a Bluetooth Receiver, an example being: https://www.amazon.co.uk/SONRU-Bluetooth-Receiver-Cancellation-Streaming/dp/B095LNGNZY/ref=sr_1_3?adgrpid=62160101696&hvadid=606171223913&hvdev=c&hvlocphy=1007444&hvnetw=g&hvqmt=e&hvrand=4974617294757291627&hvtargid=kwd-295083590140&hydadcr=24354_2303704&keywords=bluetooth+receiver&qid=1681464317&sr=8-3

Then just connect you phone to the Bluetooth Receiver and that should output via the Sonos Amp.

Edit:

Of course this would only supply audio that was stored on your phone! 

While not the most robust solution, you could configure AMP to automatically play its Line-In.

A better solution would be carrying a travel router with you.

Thanks for your responses.

So the Sonos Amp has to be connected to a network, have I understood correctly?  You cannot just attach a music source to it using cables and control the music from the music source.  Is that correct?

Line-In and the amplifier share the same shell, but they are two independent devices on the network. Line-In can be played on any player on the network, including the amplifier in the shared shell. Unlike traditional audio products, AMP’s input terminals are not wired to the amplifier. Normally, one would use a SONOS controller to select the source to play on AMP’s speaker terminals. But, you need a network to do this.

Luckily for you, there is an option to have AMP automatically select its own Line-In whenever a signal is present. This is a bit frail because you could forget to select the option before leaving the network or AMP’s Volume might be inappropriate. In this case you’d be stuck in the remote location without functioning amplifier. There is also a threshold for this auto detect. If a very quiet stretch falls below the threshold, AMP will shut down or remain in standby.

Travel router gives you full control.

Thanks Buzz, that's really helpful .  Is it the same with the Connect:Amp, please, I have one of those too?

Yes

Thanks again Buzz .