Or just block international spoofing, to allow the case you mention.

E.g. in Finland the relevant regulation (https://traficom.fi/files/media/regulation/M_28_L_EN.pdf) says (in section 10):

> Telecommunications operators must block calls terminating from the public international interface when the calling party number is clearly incorrect or the calling party number is a Finnish telephone number and the situation does not involve roaming as referred to in section 7.2.1.