Yes, been there with both the telephone avoidance and the checkout stress. It was really bad there for about 5 years or so - I got to the point where I would just sit in the car while my boyfriend ran in to do the shopping, and eventually I couldn't leave the house to even go out to the mailbox. I was diagnosed with Social Anxiety Disorder (terrible acronym - S.A.D.) which is very similar to what you're talking about - the fear of judgement and embarrassment - although it's probably worth asking a psychologist if this is what you are dealing with.
I have a theory (take this with a grain of salt!) that socially anxious people are just more attuned to picking up the subtlety in social interactions, so naturally we worry about telephone calls (where you can't see the non-verbal communication cues) and shops (where the exchange is kind of manufactured). You sound like a sensitive and self-aware person so be gentle on yourself. Only other advice I can offer is don't let yourself back out of the situations cause of the stress - that's when it gets harder. That probably sounds like the opposite of being kind to yourself, but I guess I mean do small steps - start off by just standing next to your partner at the shop counter; go to a quiet shop to practice; try to make one telephone call a day - even if it's just to ask about the bus timetable for a bus you're not going to catch. Only one step at a time though - that's being gentle with yourself. Good luck!
