You may be having 'Call Forwarding' enabled. Can you reply below with the model number of the phone you have? Also, you can try and pull out the battery while the phone is on, replace the battery and power the phone back up and retry calling it. Please not that all the above applies if it is your phone that is sending incoming calls straight to voicemail.
If it is a phone you are calling that keeps on sending you to voicemail, then the phone you are calling may be off, having no signal or it may be having call forwarding activated.
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