There exist mobile phones with dual SIM sockets for this purpose.
Apple has never made an iPhone with dual SIM sockets, but models released after 2018 (XS, XR, 11, 12 and SE-2020) have e-SIM support. If you can use the e-SIM for one mobile provider, you can put another provider’s SIM card in the socket and get the same behavior.
But, of course, this will involve having two mobile service provider accounts, which may cost more than you want to spend. VoIP is probably going to be your least-expensive option.