A voicemail issue I recently had was that Vodafone seem to think people can answer their mobile within 15 seconds or so. This default is a little short for my liking so I had a hunt around on Google and found a GSM code I could call to increase the delay to 30 seconds. I share the Vodafone one here for anyone in a similar fix.
The 121 is the number to dial for voicemail and 30 is the time delay you want before a call is diverted to voicemail. You can interchange these numbers with those for your provider.