For as long as I remember, adding $10 in pay by the minutes plan meant that you will be buying Pay By The Minute minutes at 33 1/3 cents per minute, but once you've already added $100, the service period was extended by a year.
For infrequently used phone, I seem to recall that you could leave it in daily rate plan and just pay $1 or $2 a day and pay 10c a minute and still retain it for a year. This is supported by various different links I found through T-Mobile and retailer sites.
The recent threads that are popping out and what I noticed on my account is that they're not upholding the "1 year service" that Gold Rewards customers have come to expect and Gold Rewards quietly became put behind shadows as you can't see anything about it in My Account.
So, now you can't keep Pay By The Day without paying $10 every three months to keep T-Mobile from walking off with our money for nothing?