how do i tell if the phone number that is calling me is a T-Mobile customer or...

[Curious Man]

...at&t or some other carrier? i want to use a free internet text msg provider to send text msg to a 510 area code number which is from bay area california, but to do this, i need to know if the number is from at&t, t-mobile, verizon, etc? any advice on how do i check a number online to see what is the service provider? thanks