Renting a cell phone is both convenient and necessary for keeping in touch during trips, especially if you're traveling on business or traveling in groups.

 Why?

Because roaming is extremeley expensive if you're traveling outside your region. Also, you won't necessarily get good reception or fast data while roaming. Also, if you plan on receiving calls from locals, renting a cell phone gives you a local number (e.g. 415 area code) that is easy to reach you at. To find cell phone rental agencies in San Francisco, simply search for "San Francisco cell phone rental" in your search engine of choice (Google [www.google.com], Bing [www.bing.com], etc.).

As for safety, it's important to have a number that people both locally and from home can reach you at. Incidents of missing travelers has been on the rise for years, and due diligence helps you avoid such difficult situations. If you're in a group, it's also useful to be able to reach each other while traveling and organizing.

Recomendations are always helpful. Whenever you rent a phone, you should consider options for data (e.g. smartphone), because you can often get a good deal that bundles data with typical equipment rentals.

However you search or rent, just remember to pick an agency that has a location in the city or at the airport, otherwise you'll be paying lots for shipping!