However, it could also be the reaction if your company is approaching or is currently in a busy season and needs the additional staff, or it's experiencing unusual circumstances, like a hiring freeze, which would prevent it from replacing you entirely.
What Else Should I Do When Resigning? First, out of respect and common courtesy, you would ideally tell your manager in person that you are resigning. If you can't wait for that or work remotely or some other complication prevents that, the telephone is the next best option. You can choose to follow up or accompany that discussion with a formal letter of resignation afterwards, but it would be a rare workplace that would require that formality.