Hi Jj
Some people do submit their mandatory reconsideration over the phone because they can't or don't want to write it. The problem with saying it to someone over the phone is that you have no idea if they have taken down what you said accurately - so if you can I would advise you to write it.
The Guide to PIP Appeals offers guidance, including instructions on submitting a Mandatory Reconsideration. Trying to win an MR is not easy - only 27% success rate - so do take the time to look at the guide. Have you got your PA4 - that's the assessor's report. If you have a paper-based assessment then it's a PA5 but the DWP seems to have stopped completing one. If you have the PA4, you will have a better idea of how to construct your counterargument.
benefitsandwork.co.uk/guides-for-claimants/pip BIS