Sounds like a routing problem with whatever LEC handles that particular telephone exchange. Were these always T-Mobile numbers, or were they ported from another carrier? Sometimes, a number port can result in routing problems.
You can go to »www.telcodata.us/telcodata/telco
to try to find out the original LEC for those numbers (the original LEC is still responsible for a number even if it has been ported). For example, my Vonage number is handled by XO, but my AT&T Wireless numbers are handled by Cingular.
The problem will likely need to be handled by your brother's and your mom's carrier since it is their phone numbers that can't be reached. You could of course, try Vonage support, but I suspect that would just be a frustrating wasted effort.--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.