I didn't do a telephone assessment, did a paper form.
I would say, before the call, have a bank statement to hand, details of LPA if relevant, national insurance number (often needed and I can't remember it) and the financials as above. I scanned in copies of bank statements etc and sent them to the council. You may need to check things like premium bonds etc as councils are very thorough in terms of making sure you have told them about all assets.
Keep records (if you can - e.g. letters.emails) of all interactions as I found our council process to be as clear as mud.