So I rung myself to find the same thing.
I have voicemail disabled for this exact reason because I take a while to wake up and answer the phone ( I work nights). Work leaving a message defeats the point of last minute calls and odd hours I do.
When did this start happening? It does it on a mates mobile too.
I will switch providers if this can't be defeated because I'm missing work calls. They all know to let the phone ring but that's sooo annoying Spark answering it.