This suggestion would improve T-Mobile's voicemail service.
I have a TracFone basic cell phone. TracFone does not identify the carrier, but the voicemail retrieval number, 805 637-7243, seems to belong to T-Mobile, if unknown contributors to websites that are not T-Mobile's are correct. That suggests that my cell's carrier is likely T-Mobile.
Emergency calls only are allowed in many New York City locations in the central business district in midtown Manhattan during weekday daytimes. This happens much more often than happened with my previous TracFone cell with the same number, a different voicemail retrieval number (516 606-0585), and with possibly Cingular or AT&T as its carrier.
The problem here is that, for about a year, I noticed that sometimes when I retrieve a new voicemail I discover a voicemail that was a few days old, and about which I had known nothing. Now I understand the problem: If a VM notification goes out when I can make only emergency calls, the VM notification does not arrive at my cell and, critically, is not sent again.
In theory, I could call the retrieval number just to check for the oossibility of occasional VMs. But, since I wouldn't know when to try, that would be too costly and time-consuming to be practicable.
What would be useful is if, where all calls are allowed, perhaps because I walk to another street, the T-Mobile system would tell me that a voicemail came in. T-Mobile already knows from cell tower data whether emergency calls only are allowed for someone's phone. Thus, when that status changes for the phone, T-Mobile can already deduce that. Please use that status change to send my phone an alert that a voicemail is waiting for me.
This does not require sending two notifications for one VM. You would send the second only when you did not send the first.
The advantages include that I'll retrieve voicemails much faster, I'm more likely to call people back and to call them sooner, and they might call me back after that. In my case, I'd keep someone else happier, because I'd return more calls.
If I have misdiagnosed, please let me know what solution I should try.