I recently made the switch from Sprint to T-Mobile and currently use a Samsung Galaxy S5. I ported over my old number and everything is fine except for group text messaging. For group text messages, I'm included as an additional group member. In other words, when I receive texts from the group, I see myself IN the group (I appear as my phone number). When I reply back to the group message, I send myself a text as well because it thinks I'm another group member.
I'm not sure why it does this. Any help would be appreciated!