Unfortunately, sending messages to short codes is not supported with 2nd Phone Number. This is because 2nd Phone Number utilizes virtual numbers and Voice over Internet Protocol (VoIP) technology, which differs from traditional cellular networks. Some short code services are incompatible with this technology.
The 2nd Phone Number team understands this limitation can be inconvenient, and they're actively working on finding solutions to enable shortcode support in the future. Stay tuned for updates on their progress!
In the meantime, here are some alternative options:
If you need to send a text to a specific short code, try using a traditional phone number instead of your 2nd Phone Number.