My new ISP took this to another level: if you transfer your landline phone number they send you a voice-only SIM card and a 4G router into which you're supposed

My new ISP took this to another level: if you transfer your landline phone number they send you a voice-only SIM card and a 4G router into which you're supposed to plug your landline. Calls to the landline number are redirected to the number in the SIM (with a significant delay). I wonder why they did it this way instead of implementing VoIP, especially since the all-in-one GPON/Wi-Fi router already has a landline port.