It was only a matter of time, right? Samsung couldn't possibly have expected the US versions of the Galaxy Tab to remain phone-less, especially not after giving its European counterpart the privilege.
Thanks to XDA-Developers member Jyveafk, calls are now here, at least for the T-Mobile version of the Tab. While you will look silly with a 7-inch slate held up to your face, you can always opt out for the speakerphone, which makes the Tab an excellent conference call device.
Download Odin3 v1.70 to flash.
Used P1_20100909.pit for formating. Selected 'Repartition' 'AutoReboot' 'fResetTime'. That rebooted with nothing apparently working but the filesystem structure was setup.
Download P1000XXJHA for the base rom.
Then, unchecked the 'repartion' part (as the structure is setup, don't want to screw it up, the next flashing needs to 'drop in' where the structure is.
Then, loading into the JHA.rar just downloaded, flash that and reboot.
To update to latest, download and flash P1000OXAJK1.tar
Pop in your SIM.
At this point, reboot. It'll probably show 'Emergency Calls only'
Went into the wireless/Mobile Settings/NetworkOperators and made sure it was connected to TMobile.
At that point, I made my first call and it worked. Received a call too, also working.
What the above instructions are missing is a download link. Seeing this, Artem asked for clarification on those and received this link in return, with all the downloads, including Odin3 v1.70 and P1000XXJHA firmware. Thank you, ayman07.