I work in retail for t-mobile and its very apparent a majority of t-mobile customers are getting these called and texts.
In retail we told to report any activity like this, but if we did we wouldn't have time to actually sell.
I personally believe their two reasons why t-mobile are affected
1. T-mobile numbers are very simialr and majority of the lines connected 18 months ago start 0790 (3or8), therefore very easy to call this prefix up and guess the rest.
2. T-mobile had its main growth about 20 months ago, so alot of customers are coming to upgrade now. Great time for other compaies to nick the business ahead.
The only way this will stop will be if people report this activity