Need API to create portal for downloading Contacts List onto VOIP phones

I am a self employed VOIP technician supporting HPBX customers. We use Polycom phones. Currently the phones auto provision local contacts list containing the group users within the company. Any outside contacts can only be added manually, one by one. We need to find a way to import contacts list from MicroSoft Outlook, Gmail, or other 3rd party lists using cvs or txt file . please click this link for a sample of how this API process works for a VOIP service provider that has this feature working for their customer
https://support.8x8.com/cloud-phone-service/voice/work-desktop/how-to-import-contacts-in-8x8-Work-for-Desktop