Loose, I've been frustrated by how quickly my Virgin phone switches to Voicemail and your plea triggered me to fix it! I've just researched and there is a possible solution:
For EE, type the following stars, numbers and # in to your phone keypad and press the dial button - the phone should then be updated to ring for 30 seconds before switching to voicemail:
**61*07953222222**30#
The 30 = 30 seconds and is the max you can let it ring for. You may well still have a voicemail account even though you've never used it.
Hope that works for you.
**61*07953222222**XX#