I hate all delivery apps that don't use whatever that puts the persistent thing on your iphone screen. It was made for that, dammit.
My solution to your problem is to just open the app, it defeats the purpose of notifications but I order rarely enough that it isn't a really big problem.
However just sending people straight to voicemail doesn't work in a surprisingly high number of cases - I had a voicemail from a person that was just "Hello, hello, hello". Like WTF. Explain why you were calling, thats the point of voicemail.
My solution to your problem is to just open the app, it defeats the purpose of notifications but I order rarely enough that it isn't a really big problem.
However just sending people straight to voicemail doesn't work in a surprisingly high number of cases - I had a voicemail from a person that was just "Hello, hello, hello". Like WTF. Explain why you were calling, thats the point of voicemail.