keep hours by appointment

John GrishamのThe Brethrenから取り上げる最後の表現です。
There was no Jack Argrow, Attorney-at-Law, in the Boca Raton yellow pages, and this initially caused concern. Finn Yarber frantically worked the unsecured phone, seeking directory assistance all over South Florida. When he asked for Pompano Beach, the operator said, "One moment, please," and Finn actually smiled. He scribbled down the number, then dialed it. A recorded voice said, "You've reached the law offices of Jack Argrow. Mr. Argrow keeps hours by appointment only, so please leave your name and number and a brief description of the real estate you're interested in, and we'll get in touch with you."
"keeps hours by appointment" の意味は辞書を引かなくても分かります。しかし、英英辞書を引くのが習性になってしまったので、辞書でこの表現の確認をしたかったのですが、この表現を掲載している辞書は見つかりませんでした。とは言うものの、この表現はGoogleで調べると容易に確認できるように、ごく一般的な表現である事は間違いありません。次の様な例がネット上で容易に見つかります。
・We keep hours by appointment between 9AM and 6PM. There is a $50 charge for earlier or later arrivals/returns. Please make arrangements ahead of time.
・I keep hours by appointment at my office in a medical office building on Hobson Road in Woodridge, IL.
・Great staff, quick appointments. Dentist and hygienist were thorough and very sympathetic. No problem getting an appointment, and they keep hours that work for my schedule.