Go to the contacts/ People section in your phone. Select the name(s) of the person who can't call you and instead has their calls it automatically go to voice-mail.
Once their contact details comes up, under Information Section-> click on the Send To Voicemail Option-> change it to off.
You may have accidentally turned on the option to send all calls from one person or all your contacts to voicemail.
Once this is changed back to normal, it doesn't automatically go to voicemail, instead, it rings and goes to voicemail only if you are not available to answer the call.
(this is based on HTC phone format)