My advice would be a stabilising statement like "Hello, I'm ______, here to help. So you know I have a speech impediment, so just bear with me if that flares up." It helps because it can be rhymed off but more importantly the 'finding you out' moment we all fear is owned by you until you feel comfortable not disclosing it at the start of calls. Find your own words, do it your own way. Good luck!