Hi Hampeh,
A "chat" is far too casual a word - this is an interview and you should consider it as such. Don't make the mistake that this is an informal situation - your e-mail it too casual and your must make it more formal, and you must approah this phone interview as an interview and not a friendly chat.
Let them suggest the time.
If it was only a couple days ago, then don't apologize for your "late response" - it was a weekend, after all. Do NOT delay in replying next time - they will think you are not serious about the job.
Dear Ms. Name,
Thank you for your e-mail. I was very happy to hear from you.
You are correct that for the past year, my work has been as more of a consultant, but in my prior position as an engineer at XYZ, my role was operational. I do enjoy both types of work and employing the different skill sets for each.
I would be very pleased to speak by phone at your convenience. [If there are times you cannot do this, the say something like "any afternoon at your convenience" or any times Monday, Tuesday, or Thursday of this week at your convenience" or something like that.]
I look forward to hearing from you soon.
Thank you again, and best regards,
First name last name