YouMail answers phone calls by setting up Conditional Call Forwarding with your carrier. Conditional Call Forwarding only allows unanswered, busy, and missed calls to get forwarded to your YouMail Inbox. However, all calls may be sent to voicemail when Unconditional Call Forwarding is enabled.
Check your phone's call settings to see if it might be set to only have calls from your contacts ring through. In Android, check and see if your Do Not Disturb is set up and for iPhone, check to see if "Silence Unknown Callers" is toggled on in the phone settings.
If all is off, try deactivating the call forwarding and going through the reactivation process again and see if that resets the call forwarding and activates it properly the next time around.