In her mind she might just be encouraging you to speak by putting the phone to you, go apologize to her and explain how it feels being put on the spot can cause you to stutter, all in all sounds like a normal family interaction, I've had a very similar experience where a phone was shoved in my face to speak and I stuttered throughout, it happens, its our normal, try to ease yourself into phone conversations with huge breathfuls and don't put mental effort into the phone call itself, nor the words, just speak, don't think on words, let your mind and tongue speak as one, or your mind starts panicking and your body responds by freezing your speech.