The primary issue with Phone/Email fields on Task in the API is that they are "derived" from the Contact or Lead to which the Task is related through the WhoId field and thus they are not exposed on the DescribeSObject call for Task nor is the data available in the Task entity, i.e. you can not perform SOQL on Task to retrieve this data.

In short, the best thing to do is remove these fields from the layout. This may mean you have to create a new layout for your users that access the pages through the API (Describe). If you need these values, then in each instance where derived fields are exposed you will need to hard-code the workaround to retrieve the values from the appropriate source. We have no intention of creating new derived fields going forward.

For your information the list of "derived" fields includes:

Task: Phone (source: record from WhoId)
Task: Email (source: record from WhoId)

OpportunityLineItem: ProductCode (source: record from PricebookEntryId)
OpportunityLineItem: Product2Id (source: record from PricebookEntryId)

Case: Contact Email (source: record from ContactId)
Case: Contact Fax (source: record from ContactId)
Case: Contact Phone (source: record from ContactId)
Case: Contact Mobile (source: record from ContactId)