"Some calls go directly to voicemail: When my boyfriend calls me his call goes directly to voicemail. He has a 15 and I have a 16 E this happened to him on his 15th and had to reset the phone. Is that the only option? Has anyone else had this problem? We both have T-Mobile"
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Troubleshooting Calls Going Trait to Voicemail:
A. Unblock Yourselves:
Are you two using any Security Software of sorts on your iPhones? If so, remove yourselves from being blocked. If you are blocking him through the Phone or Contacts App, then unblock yourselves.
Go Here: Block or Avoid Unwanted Calls on iPhone - Apple Support
Scroll to: Unblock a blocked contact
Screenshot:

A. Go Here: If you can't Make or Receive Calls on your iPhone - Apple Support
C. Contact your Carrier:
An all too common phenomenon, contact your Carrier, as they are the one who provide you with the service of Voicemail. See if they are booking him, unintentionally.