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The start of the work day, just before or just after lunch, or at the end of the day are the times when an office employee is most likely to be away from her desk or have her mind on other things. Give her a chance to settle in after arriving in the office and don't try to catch her as she's trying to wrap up her work and leave. Don't repeatedly call at the same time of day, if you don't get a response. Even if that's the time you've set aside for phone calls, respect the other person, and call at other times during the day. If you leave a message or catch the person on the way out, ask when you can call back at a more convenient time. You'll be surprised how many people set aside specific times for phone calls each day. Make sure you're at your desk during that time. She is probably as frustrated with playing phone tag as you are. If you demonstrate that you will be available at the specified time, she'll be encouraged to call you at that time. Tell the person you want to contact that you will call back at a specific time. This gives her the opportunity to arrange her schedule so she'll be available when you call back. If you're not a "phone person" that's okay. Set aside an hour or two each day to take and return phone calls. Be available during this time as often as possible, so your coworkers known when they can reach you. Learn if others are available to talk on the phone at specific times, and contact them during this time. If she's logged into a company-wide instant messaging or phone system, you may be able to tell when she's at her desk. You can also use these services to send a quick text message first, asking if she's available to take a phone call.
Avoid calling at inconvenient times. Call at a variety of times. Ask for the best time to call. Give the person a specific call back time. Make a presumptive appointment in your voicemail message. Set aside blocks of time each day for phone calls. Consider using the Internet to find out when the other person is at her desk.