It's best to change your phone number via your DoneDeal account so it will appear when placing your next ads on DoneDeal.
Your phone number is associated with your DoneDeal account so if you've made a mistake with your phone number you can make changes via your account. Just click "edit" in the phone number field and hit save.
Don't forget, you'll need to follow the verify phone number process again to ensure your new number appears as verified on your DoneDeal account.
If you just want to change the phone number on one specific ad, you can do this via Manage Ad. Please note, it will ONLY change for that ad and will not remember this number in the future.