If you are using AT&T towers and have an AT&T based Straight Talk phone with an AT&T phone number than it will count as mobile to mobile calling for AT&T customers. Just because you have a GSM phone doesn't necessarily mean its using AT&T. Some of the GSM phones are T-mobile based and will not count as mobile to mobile with AT&T. Most likely you are using AT&T and will be fine but its not 100%. Where I live our GSM Straight Talk phones are AT&T based with AT&T numbers so it does count as mobile to mobile to people with regular AT&T or GoPhones.