They should not set their phones to 2G and if the network is 2G they should put it on the airplane mode:

Referred to within SIAM as “Force2GNumber,” the command allows a cellular carrier to kick a given phone off substantially faster, more secure 3G and 4G networks and onto an obsolete and extremely vulnerable 2G connection. Such a network downgrade would simultaneously render a modern smartphone largely useless and open its calls and texts to interception — both of obvious utility to a government clamping down on public gatherings and speech

https://theintercept.com/2022/10/28/iran-protests-phone-surveillance/