Voicemail not answering phoneline

Have configured FreePBX 2.1.1. I have an incoming route which does the Ring All option. The ring all rings all phones for 15 seconds before going through to voicemail. However, if I don’t play an announcement to the caller, after 15 seconds it jumps to voicemail, but doesn’t answer the phone. Because the phone is still ringing (because it hasn’t picked it up), the ringall call option starts over again.

Is there a quick fix for this. I only want it to answer the phone after 15 seconds, not as soon as someone rings in.