Error when clicking on number to call

Hi All,

When I click on a phone number on contact page to call I get an Unexpected Error. This is not happening when I call from Lead page for Example. The error is showing below. Anyone has any suggestions please?

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=8.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: System.Xml.XmlException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #91AA8319Detail:
<OrganizationServiceFault xmlns:i="www.w3.org/.../XMLSchema-instance" xmlns="schemas.microsoft.com/.../Contracts">
  <ErrorCode>-2147220970</ErrorCode>
  <ErrorDetails xmlns:d2p1="schemas.datacontract.org/.../System.Collections.Generic" />
  <Message>System.Xml.XmlException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #91AA8319</Message>
  <Timestamp>2017-06-28T07:51:58.5828158Z</Timestamp>
  <InnerFault>
    <ErrorCode>-2147220970</ErrorCode>
    <ErrorDetails xmlns:d3p1="schemas.datacontract.org/.../System.Collections.Generic" />
    <Message>System.NullReferenceException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #42631391</Message>
    <Timestamp>2017-06-28T07:51:58.5828158Z</Timestamp>
    <InnerFault i:nil="true" />
    <TraceText i:nil="true" />
  </InnerFault>
  <TraceText i:nil="true" />
</OrganizationServiceFault>