Get a Direct Inward Dialing (DID) number and a VoIP host that speaks SIP. Dunno about iOS, but Android has had native SIP support built in since Android 2.3 Gingerbread.

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xethos 3 days ago | parent | next [-]

Google actually ripped VoIP support out of the native dialer several years ago

https://www.xda-developers.com/android-12-killing-native-sip...

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echelon_musk 3 days ago | parent | prev [-]

That would work for calls, thanks.

But for apps like WhatsApp, Signal, SMS, iMessage etc. they would all need their own workarounds.

	▲	Nextgrid 3 days ago \| parent \| next [-]
		But those apps don't actually care about the phone number beyond the initial registration? Once registered, they work entirely via IP and don't know nor care which SIM is in your phone. You will however have a problem with registering on those with a VoIP number - those ranges are generally blacklisted due to bad actors misusing those numbers for nefarious purposes.
	▲	westmeal 3 days ago \| parent \| prev [-]
		Could use a VNC of sorts. Maybe a Miracast receiver that dumps the screen buffer to your own web service that just spits out the frames over UDP