There are two different ways to write things down about a lead in Myphoner, and they behave differently:
Call comments (the comment box in the dialler)
The comment field you see while working a lead is saved to the lead's activity log when you disposition the call — that is, when you click one of the coloured buttons (Call back, Winner, Loser, Archive). Each comment is attached to that specific call, so over time comments move down the activity log as newer entries are added.
Use comments for what happened on a call: "spoke to reception, decision maker back Tuesday".
Tip: the comment is only saved when you disposition the lead. If you type a comment and navigate away without clicking a disposition button, it isn't stored.
Permanent lead notes (a custom field)
For information that should stay visible on the lead regardless of how many calls you make — directions, preferences, agreed terms — use a Multi-line Text field on the list:
Go to Manage -> Lists and open your list's settings.
Add a field (the + icon at the bottom of the fields list), name it e.g. "Notes", and set its type to Multi-line Text.
On any lead, open the Edit tab, fill in the field and save.
The field now shows on the lead itself, every time anyone opens it — and it supports markdown formatting.
See also: Defining and configuring fields