How do I retrieve a deleted phone number from my contacts

Is this contact in the Apple Contacts app on your iPhone? If so, do you sync it to iCloud? If you do, you should be able to restore this contact, including the phone number, from your iCloud account.

This would be easiest performed from a Mac:

• Open iCloud.com in a browser, and then, log in.
• Select Settings from the dropdown menu located beneath your username in the upper right corner.
• In the next window, select Restore Contacts in the Advanced section located in the lower left window.
• Select the date from those offered in the archive list.