a beautiful thing about the gms system is that you can totally change your call management for voice, fax, data -- forwarding calls to *different* numbers, even for various scenarios (busy, unreachable, etc).
typically you'll want everything to go to the 'system' voicemail - which is really a call center provided by your service provider. this is what most people have, just use the stock settings - and system offered by the provider.
but what i really want is voicemail delivered over the phone, and by email. plus, say i go on vacation, i can have all my calls routed to my house instead, or business partner - not ring my phone. whatever the circumstance, it's up to you. the flexibility is built-in.
getting my voicemail over email is great, especially convenient if i'm working i'm at my desk. if they've already been downloaded i can review them, say on the plane -- or say at burningman where there's no cell tower or phone line in sight ('cept for voip -- and i really don't want to see cell phones at burningman either ~but~ it's nice to get messages).
right now i'm testing out forwarding my calls to a service that let's me check my voicemail over the phone, web, and email. this would also be useful while traveling -- looking for ways to avoiding international roaming too. i already use the t-mobile 'trunk' line to check voicemail from a landline without using long distance or ringing my phone (nice for checking mail from home or office) -- but delivery over email gives me more options here, especially when travelling. this also gives me a better call log, and archival features.
email serves as better long term storage -- say while your sailing around the world, even being accessible over shortwave packet radio. ok more realistic over satellite, but you get the point. t-mobile will only store you voicemail for so many weeks, saved or not.
even just email delivery, would be nice feature for t-mobile to add -- not to mention calls logs (including offline calls!) -- i don't imagine they'll add those features any time soon. it be easy enough, but customers aren't demanding it.
cool thing, you can totally reprogram your 'voicemail' button, i still get to use the built-in shortcuts keys. though i have to type more digits than my phone will store - like my pin - not optimal but it works well enough. there's the added expense of handling voicemail using another provider, but it's just a few dollars a month -- not bad really. i'm still testing things out. and seeing how many more minutes this will adds to my plan usage.
one other quirk - in my case, the caller id stamped out on the email isn't always right (forwarding number v. original caller). some of the emails simply appear as calls from my cellphone. but this is something my message center could address. who knows, might even be service providers out there that has this all worked out, i haven't even thought to look - probably a lot of options.
offhand i'm guessing i'll use the stock t-mobile voicemail service on a day to day basis (no email) - but when traveling i'll definitely switch -- given all the advantages that comes with email delivery.
the fact it works at all is a step in the right direction !