Last time i bought prepaid SIM in Belarus with MTS two years ago, I was told, it was on the contract, and i could verify by use of course, that the ones sold to non-resident foreigners:
- work only in Belarus, dead outside
- don't expire
Expiration i haven't checked as i didn't return since. Mute outside Belarus yes, i tried it in Russia, no roaming. Interesting to know if it has changed.