Some operators do this by the SIM, Sierra Wireless as a virtual operator for example has a smart SIM that doles out IMSIs to the modem to use depending on the roaming and signal parameters. This way they can provide global access without operator roaming - if you go to another country the SIM will give you an IMSI associated with a local subscription.

So I guess you can just use this tech to continuously rotate IMSIs among a large pool every few hours if you'd want.