It's been discussed in several threads here that alerts sent via SMS to T-Mobile subscribers are received from random numbers, and therefore are displayed in multiple text threads. T-mobile has been pretty useless when it comes to acknowledging or correcting this.
There is a work around that is ALMOST compatible with SimpliSafe. If the message is sent to "+1 then the phone number at tmomail.net" it will display the sender's address (noreply@simplisafe.com) instead of a random number. For example, if you phone number is 212-555-1234, you would send an email to +12125551234@tmomail.net.