All good advice so far.....

Don't talk 'down' to them like they're stupid,
But don't use those huge technical words either,
Try to find a middle ground.

Be polite.
No 'ummm's', or 'you knows'.
Be confident in your ability.
Don't BS, if you don't know something,
say 'I don't know but I'll be happy to find out for you.

If you're having a bad day don't gripe and groan on the phone.
Always speak positively about your job and the company in general.

Be genuine.
Smile! It will come through in your voice.