the phone interview won't be challenging. just be ready to discuss your background, listen to them explain a bunch of stuff, and i would consider having some common questions for them prepared.

tax people primarily work from the office. depending on what line of tax you are in, i would expect to be working from the office most of the time with some limited time at the client site.

in terms of what you will be doing, it depends once again on which tax practice you are in. if you are going for lead tax services, you will be doing "compliance" work, which is a fancy way of saying, tax return preparation. if you join another tax group, (international tax, M&A tax, state and local tax, etc.) you day to day duties will vary.

honestly, if you don't have an interest in tax or know anything about tax, it might be better to go in the audit direction. if they only offer you the chance of a tax internship, take it, and then consider trying to switch over at the end of the internship.