It always means that you must sacrifice something very dear to you when His call comes.
When we think deeply about the problem of life, there is no path in the world, whether spiritual or material, which we can tread successfully without a sacrifice. Sometimes the sacrifice is great, and sometimes small; sometimes the sacrifice is made first, before achieving success, and sometimes afterwards. As sacrifice is necessary in life, it is made by everyone in some form or other, but when it is made willingly, it turns into a virtue. The greater the ideal, the greater the sacrifice it demands… sacrifice of a possession is the first step; the next one is self-sacrifice, which was the inner note of the religion of Jesus Christ. Charity, generosity, even tolerance and forbearance, are a kind of sacrifice, and every sacrifice in life, in whatever form, means a step towards the goal of every soul. – http://wahiduddin.net/mv2/IX/IX_22.htm
I gave up my music because I had received from it all that I had to receive. To serve God one must sacrifice what is dearest to one; and so I sacrificed my music. I had composed songs; I sang and played the vina; and practicing this music I arrived at a stage where I touched the Music of the Spheres. Then every soul became for me a musical note, and all life became music. Inspired by it I spoke to the people, and those who were attracted by my words listened to them, instead of listening to my songs.
Now, if I do anything, it is to tune souls instead of instruments; to harmonize people instead of notes. If there is anything in my philosophy, it is the law of harmony: that one must put oneself in harmony with oneself and with others. I have found in every word a certain musical value, a melody in every thought, harmony in every feeling; and I have tried to interpret the same thing, with clear and simple words, to those who used to listen to my music. I played the vina until my heart turned into this very instrument; then I offered this instrument to the divine Musician, the only musician existing. Since then I have become His flute; and when He chooses, He plays His music. The people give me credit for this music, which in reality is not due to me but to the Musician who plays on His own instrument.
Hazrat Inayat Khan – http://wahiduddin.net/mv2/II/II_0.htm