I would think the swatters would just buy a cheap burner phone with cash for these calls. They don't even need a SIM card; in the US, at least, a SIM isn't required to make emergency calls.
(As a PSA for anyone with an emergency phone in the UK) You now need a valid SIM to ring 999 in the UK as they were getting too many nuisance calls - but any network will roam the call if reception is bad on your own one.
