Not sure if this is what you mean, but I think so: Go to settings > Call settings > then click Voicemail (the first entry). A window pops up that lets you enter your voicemail number - the default is #86, so it seems like this is the thing you're trying to change. I hope this is what you're looking for!
Don't need to do that at all with You Mail.......thats the beauty of it.....just tap on youmail, loads message, listen. You do get a notification when you get a VM, based on your preferences, naturally.