I’d check the Audit Trail under the “More” menu in the admin portal. 
It will show changes made that might cause this. 

Also worth double checking - any custom rules and the call handling of the phone number - for example it’s not clear incoming calls to what sort of number are going to voicemail:

• direct number of your user extension
• your company main number
• additional number assigned to a call queue,etc.

Depending on what sort of number is sending calls to voicemail, there are different things to check.

Great suggestion @Problem Solver! 
@C. Landgren, please mark his comment as the best answer if this helped you. Thank you!