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If you are with the person, try taking their phone. Note that it's best to be open and flirty about it since you could get into trouble if you take their phone secretly or against their wishes.  Send yourself a text message from their phone, and add your contact to their list. Look up their number in their phone settings. If you have a mutual friend, simply ask them for the person's number instead. This way, you get the number without having to be so up-front about it. If you have a phone book, school directory, or archive of coworker phone numbers, look up their number. Use whatever method you have available to find their number.
Take their phone. Ask someone else for their number. Look it up in a directory.