You don't need a phone number if you don't need to be called on one, and you don't even need to be actively tracked in the home location database if you don't need to be reached at low latency at all. Think low latency=incoming call, no loner low latency=hourly email check. The remaining overhead is just renewing the sim once it degrades and keeping database space to allow it to associate if it turns the gsm module on.