The delay before voicemail is set by the network, you can either ring them to get it set to the maximum (30 seconds) or if you are feeling bold you can do it yourself but you need to know what your voicemail divert number is, on Orange for instance it is 07973100123 and in that case the command that you enter is **11*07973100123**30# and then press the call key. This will set all voice calls to a 30 second delay to Orange's voicemail. Messing with network codes can be risky if you don't know what you're doing so it's at your own risk :-)