Does she have a signal? If her phone is not getting adequate service, it would go straight to voicemail. Another possibility is that her phone has a feature to automatically forward some contacts to voicemail, and it was set up for your number.
In addition to what spottedcatfish suggested, I would recommend ensuring call forwarding is deactivated. To deactivate call forwarding dial *73 and send, you will hear a short beep, then end the call. Hopefully these steps resolve your issues.. Please advise if you require further assistance.
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