Stop beating yourself up, you did good man. Do me a favor write down 5 things your grateful for today? I'm grateful for feeling your pain bro, I'm stutter just like you and I go through the same things. I am grateful to know I am not the only one. Most of the time people feel sorry for us, they dont think of all the negative shit our brains tell us. Most of the time people want to help us. Think positive, you could always write the lady an email and say "I left a message for you, her is my phone #. I like to apologize if I stammered on the phone call. I'm really excited about this position because I feel I am a good fit because of ABC, etc." That shows your have humility, tell them why you are qualified etc.