I've been a tmobile customer for about 12 years now... Well, not technically.
I'll start this by saying that I don't fully know the whole history of T-Mobile. I assumed at the time that these changes were just company name-changes, but I'm sure most of them were one company getting bought by the next...
When I was 11, my brother was 16 and got his first cell. It was with a company called PowerTel.
By the time I turned 16, I didn't get a new cell, I got my brother's old one, and his old number. By then, I think the company was called VoiceStream.
I don't remember when, but the company eventually went to T-Mobile.
And, as a matter of fact, I think, somewhere in there there was 1 more company name-change.
So the line I have has been active for about 12 years. Not always with "T-Mobile" and not always mine, but has been mine for 7 years.
Pretty crazy, really. And the "I've had this line active for 12 years!" line has really helped me with phone upgrades, etc., in the past!