On the woman’s voice in the eyes of Shari`ah, the Standing Committee for Islamic Research and Ifta’, state,

The woman’s voice is not ‘awrah in principle, since women have been used to complain to the Prophet (peace and blessings be upon him), and ask him about Islamic matters. The same was too with the Rightly-Guided Caliphs (may Allah be pleased with them) and Muslim rulers after them.

Also, women would greet non-mahram men with salams and return greetings, and none of the imams of Islam denounced them for that. What is not permissible for a woman is to speak in a soft or alluring voice, because Allah says: 

(O wives of the Prophet! You are not like any other women. If you keep your duty (to Allaah), then be not soft in speech, lest he in whose heart is a disease (of hypocrisy, or evil desire for adultery) should be moved with desire, but speak in an honourable manner.) (Al-Ahzab, 33:32)

This is because men may be tempted by that, as is indicated by this verse.

Sheikh Yusuf Al-Qaradawi, the head of the European Council for Fatwa and Research (ECFR), and the president of the International Union for Muslim Scholars (IUMS), states the following:

The only prohibition is the female talking softly and flirting in a manner meant to excite and tempt the male. This is expressed in the Qur’an as complacent speech which Allah mentions in surat Al-Ahzab, verse 32, (‘O wives of the Prophet! You are not like any of the other women: If you do fear Allah, be not too complaisance of speech, lest one in whose heart is a disease should be moved with desire: but speak you a speech that is just.)”

Based on the above, there is nothing wrong with a woman recording a voicemail by her voice provided that her recording is not said in an exciting or alluring manner that can tempt others.