It sounds a wonderful last line and conjures up all sorts of possible scenarios; I can understand why you want to be able to use it. I wonder whether you can, though.If, as Daed suggests, you can prove it works by testing turning off your own voicemail, fine, but I suspect phone companies have a system that will automatically disconnect a call that goes unanswered – to free up the bandwidth. (Landlines operators do this, too.)
Could you come up with an equally good last line that involved the call transferring to a voicemail that would never be answered?
Alternatively, you could simply establish earlier in the story that this particular mobile, exasperatingly, refused to go to voicemail – it just keeps on ringing and ringing; nobody can fix it. 🙂