Is it permissible to use the phrase "To God we belong, and to Him we are devotedly attached/in love" (إنا لله وإنا إليه عاشقون) and similar expressions? And is it correct to describe God as longing to hear the voice of His servant?

It is not permissible to attribute the words "ishq" (passionate love/ardent longing) or "shawq" (yearning/craving) to Allah Almighty. This is because they have not been mentioned in the sacred texts, and due to the meanings they imply which are not befitting of Allah, Exalted be He. "Ishq" is an excessive love associated with desire or change, and "shawq" is for one who is absent, while Allah is transcendent above all of that. The most appropriate and perfect word to use is "mahabbah" (love), which has been mentioned in the Quran and Sunnah.