The T-Mobile SIM that we have in stock are activated with numbers allocated direct from T-Mobile.
In the United States (as with most other countries) when telephone numbers expire they are put back into circulation. All phone companies do this, otherwise the numbers would run out. Unfortunately, some people have recieved calls and text messages intended for people who previously had their phone number, but note that this can happen anywhere and is not unique to T-Mobile.
Rest assured that the numbers are direct allocations from T-Mobile, and we have now requested that they come from different area codes to reduce the chance they have a recycled number.