Hello juli118,
Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with answering calls with Siri. To start, check out this article about what Siri can do on iPhone: Find out what Siri can do on iPhone
This article goes over making and answering calls on iPhone with Siri: Announce calls, messages, and more with Siri on iPhone
"Siri can announce calls and notifications from apps like Messages on supported headphones and when using CarPlay. You can answer or reply using your voice without needing to say “Hey Siri.”"
If she uses AirPods, this link with help with how to answer calls: Make and answer calls with AirPods on iPhone
We hope this helps.
Cheers!