Hello @sipper,
I understand your point of view. Even with emails there is also ne guaranties when you use a secure email provider that the whole route between servers is encrypted, and it is true for incoming and outgoing emails. It is more and more so, but not everywhere yet. Also, SSL is secure communication between servers, but unless encrypted, emails are still clear text and often kept on servers as such.
I use (among others) https://mxroute.com. They are a “host” email services. (There is a deal still available at 30$ every 3 years for 25GB storage.) I did not try it but you can modify spam assassin conf rules to mark as spam any email having the SMS phone number you do not want, then send these email to trash. But then you have to forward the emails, etc. This will not be an easy to setup just for what you want to do.
On voip.ms side, since this is not yet possible to block a phone number, you can indeed stop using SMS for a while on that number and get another number for SMS.
Knowing that your situation is most probably temporary, I would try to find a free solution—like GMail or any other that can do the same.
Maybe you could ask Michael Kourlas if the block feature on his Voip.ms SMS application is on the radar? But I think it will not be done soon as this is open since 2017. There is an issue on this:
This would be a nice feature in fact. It could automatically delete undesired SMS instead of displaying them. This is not totally block, but “auto-delete” feature.
See this post for the links to his app and GitHub:
Kind regards! 