While it may seem simpler to call, you may be able to say in a letter what you wouldn't dare say on the phone or in person. And your letter may well be cherished and saved for years. Write to a stranger. You'd be amazed how many people out there need a friend. Write to your congresspeople.
Participation is what makes democracy work.
If you only communicate by voting, your representatives might not get your message right. Write for human rights. Let them know you know what they're doing. Write to the media. Be the good news.