You can show your gratitude for the long-lasting favorable relationship!
When you write a Japanese business email to your customer (client), the sentence often begin with a greeting phrase "Itsumo osewani natte orimasu," meaning that "I am grateful of your kind service, always."
And the same phrase can be used when you receive a phone call from Japanese client. The phrase is not something that you would use to your friends, but is a very elegant way of showing your gratitude to your customer and business partner, as it expresses your good will for the long lasting relationship by including a Japanese word "itsumo" which means "always" or "whenever."
You can also combine this "itsumo" word with a phrase "Arigatou gozaimasu" (meaning "thank you") and then it becomes "I am thankful of you for all you do for me everyday." If you want to say that to your friend, then it may be "Itsumo arigato-ne."
Hope these words help you build a long lasting relationship and friendship with your customers and friends.