Hi,
I'm not following you. HikaShop has no phone number field which can be saved in the Joomla profile.
The custom fields of HikaShop can only save themselves in HikaShop tables.
A custom user field would be saved in hikashop_user, an address custom field would be saved in hikashop_address.
If you want to save it somewhere else and supposing that you're using the HikaShop registration form, you would need to develop a small plugin, implement in it a hikashop trigger like onAfterUserCreate or onAfterAddressCreate to take the value of the custom field and add it in Joomla where needed with your custom code.