Hi,
Not being able to talk is common with anxiety ie we have fight, flight, freeze responses to what our brains perceive as a threat. Our not able to talk is just an ultra freeze state where our brain is protecting us.
I know exactly what you mean. Trying to verbalise when in this state (definitely my anxiety) is guaranteed if it comes out, it's slow, wobbly, jerky, emotional. I especially experience it if I have to ring up a business and especially a govt department, but it could also just be to order take away. Asking for help to find something in a shop is trouble too. Body freezes and intense pressure inside knowing it's building up trying to come out of my mouth.
I manage better now by saying up front to the shop person or on the phone that I suffer from anxiety disorder and it's difficult for me to verbalise without sounding upset. If i sound upset please ignore it. That's my life. Just talk normally to me and ignore if I sound anxious or distressed.
Of course it isn't always that long, but once said, it takes away the fear of what they think of me etc and I continue on sometimes with very marked speech impediment or in tears. Breathing is ragged and an effort to do so.
I must admit, since I started warning them up front and that I was capable of completing the task, I have 99% found people respond to this is a postive way and let me go at my own pace. Some I can tell may feel uncomfortable, mainly because they're not sure if they are doing the right thing. Works WONDERS in a shop as they go and search what you are looking for. Then again, I'm old, but I believe most will treat you with empathy and respect since you were brave enough to bring it up.
If you really can't get a word out to give that heads up to them, if it was me, I would carry a nice card with the above expalanation if you know you just need some time to start (put that on there too). If you REALLY can't talk carry a notebook and write down what you need to say. You can just say sorry, you're unable to talk and they'll think you literally can't. You could even have a small note / letter prepared to give to them if you need something specific.
My experience has been voice coming out very emotional and body shaking to start with but years later, it comes naturally and although I still may warn them, I don't suffer as much because I know I don't have to be perfect. A 'thank you for understanding" at the end feels good too.
Being prepared is the key so you can call on whatever tool you have to be able to communicate even if it's not how we would like. People nowadays understand more than they did 10 years ago and often give you extra help.
Good luck