I do have social anxiety which doesn't help first of all.
I do a daily walk to get my groceries and every time the big issue seller, charity collectors etc outside two of the shops will keep trying to get my attention as I go in. I pretend to be on the phone as I find it difficult to converse with people. They will still keep saying hello, do you want a big issue, are you interested in this charity etc.
If someone is already talking on the phone (ok I am not really) but what is the point of keeping on trying to strike up a conversation with them? They are unlikely to cut the call just to respond to a charity collector, big issue seller.