An enhancement has been submitted to investigate possible workarounds or alternate configuration options. This is under review by Product Management, however, no ETA is available at this time.
The Alternate Number field can be used as a workaround to specify the number in the correct E.164 format (ie. +18003069329), if the Mobile Number field in Active Directory is set to use a different format. This avoids the need to modify the users information in Active Directory.