It's a matter of simple courtesy to not intrude on those working nearby by taking phone calls in the coffee shop. Folks have a tendency to talk louder than normal on such calls, and they can disturb those nearby. It's also not that uncommon for phone callers to discuss topics they don't really want others to hear.
Yesterday I worked a couple of hours in the local coffee shop, and one suit in particular made call after call at the table next to me. He was definitely what you would call a 'loud talker', and every call he made stopped everyone nearby.
I doubt the major laptop manufacturer he works for, who I will not identify, had any idea these calls were transpiring in a public venue. I am positive they didn't want the fact that one of their upcoming laptops, identified by name, is going to be significantly delayed. I am absolutely certain they would not be happy that insufficient heat dissipation was shared as the cause of the delay.
I am not located in Silicon Valley and I still heard this tidbit of confidential information. I can only imagine what secrets are shared in Silicon Valley coffee shops.
So think twice before you disturb fellow patrons in a public venue with phone calls. Do the courteous thing and take the call outside. You might keep sensitive information private on top of being a good neighbor.