Voicemail interruption / detection

Hi,

I'm looking for a way to detect in general voicemail systems and/ or to interrupt the welcome message.

Most voicemail systeems have a welcome prompt followed by a beep. What is the duration / frequency of this beep in general?

Is there a dtmf / tone to interrupt the welcome message in general for most voicemail systems? I know some facilitate using the # or another dtmf, but this is not general and I don't know the telco's to enable specific dtmf interruption for a telco.

Thanks for your time.