Nokia Lumia 820 voicemail is always engaged.

[WPCentral Question]

The voicemail on my Nokia Lumia 820 is not working. I have set the Voicemail to ON, but when people leave a message. I cannot pick up my voicemail messages. The number (121) is permanently engaged. I have had the phone for months now, and I have not been able to pick up my voicemails once.
I am with o2. I would like to change the number of rings before forwarding to voicemail.

 

[Ryan ODonnell]

O2 voicemail is your own mobile number. Ring your own number (07xxxxxxxxx) and it should get in your voicemail

 

[gomezz]

Voicemail on O2 is 901 not 121.