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T-Mobile is a network type that your iweargps watch should work with.
But the network plan you use has to support GSM G2.
Your plan must have NO Voicemail, and must have Call Display capability.
You need to set the SOS Emergency Number before the iweargps will be usable.
*If* problems persist, you may have a defective iweargps watch device, in which case you can turn it in as defective for an exchange for a different iweargps device of equal value and capability.