The easiest thing to do is to put it on automatic selection.  If you
are a T-Mobile subscriber it's more than likely you will *only* be
able to register on the T-Mobile network.  The only time you can use
other networks is if your home carrier (in your case T-Mobile) has a
roaming agreement with those other networks.  If no roaming agreement
exists you will be denied access to those other networks.  Sometimes
if T-Mobile (or another network) does not have native coverage in an
area they will arrange to roam on another network.  This is called a
roaming agreement.  If there is none you cannot use any other network
other than your own.

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Either completely fail to work, or give you high roaming rates, depending on
the roaming agreements T-Mobile has with these other carriers.

miguel