Personally, when I schedule a phone interview for myself, I try to be at home, with a charged cell phone, no interruptions, with a glass of water handy. I dress up (no sweatpants!) because I feel more professional and confident in work clothes. I stand up for the interview so that my voice is loud and clear, and I make an effort to smile while speaking so that my enthusiasm and passion for my work is heard through the phone.
However, circumstances are not always ideal and sometimes you need to take the call from your car or a local coffee shop. Maybe you're on lunch break from your current job. As a recruiter, I try to understand that you are taking time from your schedule to speak with me about the role, but there are limits to my patience. If there is extreme background noise, I will ask you to relocate. If you sound like you're in your cubicle, muffling your voice so that your manager can't hear the conversation, I will ask to reschedule to a time where you can speak more freely.
I've had calls from people who are on trains, in airports, and at home (dogs barking, children crying). But my favorite story is when I was on a call for someone interviewing for a Warehouse Associate position. The interview went fairly well, took about 20 minutes and I was satisfied enough to bring the candidate in for an on-site interview. I started relaying address and date availability information, when I was interrupted "Woah hold on, I'm sorry Miss," the candidate said, cutting me off. "I'm in the tub. Can you email this to me?" Alrighty buddy, sure thing. Pass the soap.