[tcrhelpforsms]

I suspect you are misunderstanding the use case here. If I am understanding you correctly, you are proposing solution using single phone. As such, only one person can have the phone with them at any given time.

Also, a dumb phone doesn't really help with the need for SMS and MMS.

Update: I had indeed misunderstood and the proposal was to use a cloned SIM and two phones with dual SIM support. However, as discussed deeper in the thread this still seems to have flaws.

[systemz]

Yes, I was suggesting just that. In that case, at least in EU there is an option when one number can be used on more than one SIM card. Then using second SIM slot on both phones you have access to that number. Incoming SMS goes to both number I think. Calls can be answered by any phone but faster to answer wins.

[hluska]

It works in many providers in Canada too.

Our mobile networks just recently upgraded from two tin cans and a really long string so I feel like if we support it, it must be pretty close to universal.

[tcrhelpforsms]

I have not heard of such a thing in the US, but I will investigate.

So while that would work for calls, does that also behave as expected for SMS and MMS?

[hluska]

No, you can multiple sims in one phone, at least in the Apple world. iPhone has supported dual sim since 13 and eSIM forever. I don’t know about Android but I’m sure you can research that with a search like “{ your partner’s device mfg + model } and dual sim support”.

[tcrhelpforsms]

I'll investigate this, but I'm kind of surprised this use case would be even allowed? How does this wind up being different than a SIM cloning attack?