Dumbphone and number from standard cell provider seems like a simple fix. No iOS or Android necessary.
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.
So while that would work for calls, does that also behave as expected for SMS and MMS?
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.