Is it permissible for a man to speak with a non-mahram woman over the phone, and is the phone considered a hijab in this case?

It is permissible for a man and a woman to speak to each other if there is no Shariah-forbidden aspect involved, such as fear of temptation or softening of speech. This applies even more so to speaking on the phone, provided that the conversation is limited to what is necessary and conducted in an appropriate manner, without softening the voice, in accordance with His (the Almighty's) saying: "Then do not be soft in speech, lest he in whose heart is disease should covet, but speak with appropriate speech."