Query extension by phone number

I have to write an app that sets a call forwarding destination for a user. In order to call the API I need an extension ID.

/restapi/v1.0/account/~/extension/'+extId+'/forwarding-number

However, all I have is the users DDI phone number to query with. So it would appear I have to first get a complete list of all phone numbers on the system and this will include their extension IDs.

/restapi/v1.0/account/~/phone-number

From there, I can use this list to lookup the extension ID for a specific phone number. I have over 25K users on the system. This takes time and puts load on the system. Is there no way to query just one phone number and get just it's details?

Thanks.