You can dial 121 or 123 from your phone to be connected to your voicemail or press and hold 1 from your phone.
If you're abroad, then you'll likely need to press and hold 1 to be able to access your voicemail.
To access your voicemail from another phone, dial 07782 333 123 and follow the instructions. You'll need to have set-up and know your voicemail PIN in order to access your voicemail from another phone. To set-up the PIN dial 121, or press and hold 1 on your phone and then press 4, 2, 1 and you should be asked to set-up a PIN