プレフィックスの電話番号に＃を追加し、連絡先の予備の電話番号として保存します。
Add # to prefix phone number and save as secondary phone number in contact.
For example:
Contact abc has one number only 088 088 0888
Using your app, you can create a second number for the same contact but prefixing it with a #. Then Contact Abc has 2 numbers 088 088 0888 and #088 088 0888