。
She covered the phone's mouthpiece with her hand.
Literal
She [topic-は] phone [possessive-の] mouthpiece [object-を] hand [with-で] covered.
送話口 ('mouthpiece,' literally 'sending-talk-opening') is a technical term for the speaking end of a phone handset — the kind of specific vocabulary that might come up in descriptive writing. おおった (覆った) means 'to cover.' The scene implies she was trying to muffle her voice during a conversation.