Monthly Fixed expenses and Variable expenses

I have struggled to define what's fixed cost vs what's variable cost....Example: Cell phone..is that a fixed or variable cost?
I have signed up for a 2 year contract and if I do not have additional charges then the monthly cost would be my fixed cost..Can I live without it? Sure..I can..but is it feasible?..probably not..

I want to eliminate whatever fixed cost I can and limit/lower the variable cost or keep them predictable..This will help me target the retirment amount that I need to aim for and also determine adequate withdrawal amount to maintain my standard of living..