Praise be to Allah.
It is not permissible for a man or a woman to work in places where women mix with men. The evils of mixing are many. It is one of the most effective ways used by the shaytaan to make the Muslim fall into sin and immorality. Hence Islam blocks the ways that may cause a Muslim to fall into anything haraam.
We have discussed the prohibition on mixing in the answer to question no. 1200, where we mention incidents of female workers who have been exposed to annoyance and harassment. This confirms that the Islamic prohibition on mixing is what will protect a woman’s modesty and chastity, and it is what protects a man from allowing his gaze to wander, and protects him from bad consequences.
There is no reason why a woman should not work if she is with other women like her, or she is alone and answers calls from customers or calls them to offer the company’s products, so long as she observes proper etiquette when speaking and does not speak in an alluring tone to men, because Allaah says (interpretation of the meaning):
“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”
See also the answer to question no. 27304, which discusses the ruling on how to address women at work, and the answer to question no. 20140, which explains the ruling on a woman working as a secretary for men in the workplace.
And Allaah knows best.