[Q] SMS receives but fails to send. Swap SIM and it works. topic

Background: I bought an S6 three weeks ago, and swapped SIM cards with an iPhone 6. Both phones are on AT&T. Everything worked fine.

So my configuration was:

iPhone 6: New SIM
S6: Old SIM

This weekend, my S6 decided to stop sending SMS. I still continue to receive text messages, but cannot reply. Factory reset did not help.

I swapped the SIM cards back, and both phones work as expected. To be clear, the S6 with my new SIM card is able to send SMS.

What could possibly be wrong? It seems like something with my account.

TIA...