Get GUID of Contact Entity Record from an external JS Application

I am using Xrm.page library to open existing contact entity records. For this I need to pass GUID as a parameter:

Xrm.Utility.openEntityForm("contact", GUID);

I know the business phone number i.e. telephone1 of the record, so can I use that number to somehow extract the GUID of the corresponding record? Important thing is that I am writing and running this code outside the CRM, so I am not using web resources, I need some way of using Xrm.page which does not involve usage of web resources.