いつもお世話になっております。
Itsumo osewa ni nate orimasu.
We appreciate all that you’ve done for us.

One of the relatively few Japanese phrases widely recognized outside Japan is moshi-moshi, which, among its other functions, is frequently used as a sort of greeting when one answers the telephone. A mere moshi-moshi will not suffice, however, when you answer the office phone. Let’s say it’s ten in the morning and the phone rings at the trading firm Asahi Shoji. You answer:

おはようございます。アサヒ商事です。
Ohayou gozaimasu. Asahi Shouji desu.
Good morning, Asahi Shouji.

or

はい、アサヒ商事でございます。
Hai, Asahi Shouji de gozaimasu.
Hello, Asahi Shouji.

It’s essential to promptly state the name of the firm. Of the two alternatives shown above, the first is often preferred, since the initial greeting ohayou gozaimasu is thought to strike the sort of cheerful, resonant note that reflects a positive and businesslike atmosphere (not in the afternoon, though). Some firms have rules stating that callers must be greeted with an apology if the phone has rung more than, say, five times:

お待たせいたしました。アサヒ商事でございます。
Omatase itashimashita. Asahi Shouji de gozaimasu.
Asahi Shouji. I’m sorry to keep you waiting.

Generally, the caller’s first duty is to identify herself, and that is your cue to say:

いつもお世話になっております。
Itsumo osewa ni natte orimasu.
We appreciate all that you’ve done for us.

A variation frequently used by speakers of the Kansai dialect prevalent in western Honshu is:

毎度お世話になっております。
Maido osewa ni natte orimasu.
We appreciate all that you’ve done for us.

To the analytical mind, it may seem inordinately gushy, even by Japanese standards, to use the words itsumo (literally, “always”) and maido (“every time”) with every Tom, Dick, and Hiroshi who happens to call. After all, the caller could be anyone, including a corporate extortionist or the disgruntled husband of the boss’s mistress. Still, it doesn’t pay to be too literal-minded about these things, and none but the most utterly cantankerous caller is likely to object to gratuitous words of appreciation.

Now and then you may encounter a caller who fails to disclose his name at the start of the conversation. In that case, you have to inquire:

失礼ですが、お名前を ……
Shitsurei desu ga, onamae o …
I’m sorry. May I have your name, please?

or

失礼ですが、どちら様でしょうか。
Shitsurei desu ga, dochira-sama deshou ka.
I’m sorry. May I ask who’s calling?

As noted elsewhere (see the section entitled “Pardon me for asking”) the message to be conveyed is so utterly predictable from the context that the expressions above are very frequently reduced to an abbreviation: the speaker utters only the introductory phrase and leaves the question itself unspoken.

失礼ですが ……
Shitsurei desu ga …
I’m sorry …

In some cases, such as when the caller has seized control of the conversation and is off and running before you get the chance to ask who’s on the other end, it can be more politic (and more fun) to take a euphemistic approach, one that has a fairly exact counterpart in English:

恐れ入ります。お名前が聞きとれませんでしたので、もう一度お願いいたします。
Osoreirimasu. Onamae ga kikitoremasen deshita no de, mou ichido onegai itashimasu.
I’m terribly sorry. I’m afraid I didn’t quite catch your name. Could you say it again, please?

This not only requires the caller to identity himself, but also subtly reminds him that he should have done so at the outset.

– Source: A handbook of common Japanese phrases –