Yes, looks like you can call 0414121121 from a landline to access your TPG mobile voicemail – as long as your TPG mobile service is on the Vodafone network. (If you’re on an old plan on the Optus network, this won’t work.)
First, you need to set up your voicemail from your mobile phone. Dial 121 (standard call charges apply) to set up your voicemail and your voicemail security code of 4-10 digits. You’ll be asked for this code when you're checking messages from another phone.
Then, check your voicemail by calling 0414121121 from a landline. You’ll need to enter your mobile number and the voicemail security code you set up.