Prevent Apple Watch calls from switching to iPhone speaker

I am trying to enable my sister in law to use apple watch Ultra 4 combined with Iphone 17e. Due to ALS, she cannot use her hands which means that we need to rely on voice control only. The most urgent situation is that she can reach out for help using Siri to make the relevant calls and then continue talking through the watch. This is functional. But, when we try to call her, she cannot take the call. We tried automated pickup, which works, but then the system automatically switches to the phone speaker and she cannot use the watch to talk. Is there any way to prevent this automatic switching to the phone speaker?