The last time I looked on the VoIP.ms site about email to SMS, it said they did not support sending SMS to a short code. For this reason I left one number of mine that sometimes receives a message from a short code that needs a reply on Telnyx.
It happened that in the last few days I received two unexpected and unwanted SMS messages on a VoIP.ms number I have set up with SMS to email. One was from a clinic where I had had a medical appointment, and the other was from Press Ganey, a company they hire for patient satisfaction surveys. The message that appeared to come from the clinic itself was also about a survey. In any case, I replied to each email without changing anything (i.e., the recipient or the subject) with a message that simply read STOP, and in each case I received a reply that they wouldn’t send me further messages.
I guess this does not mean I will be able to reply to messages from any short code, but I might just port that last number from Telnyx to VoIP.ms. It would be nice to hear from someone in the company as to whether they have improved their support for receiving from and sending to short codes.