The Light Phone II can be used as a secondary phone in a few different ways depending on your needs.
Swapping SIM Card: If you have a primary phone setup with a Light Phone II supported carrier then you may be able to simply put your current SIM card in your Light Phone whenever you want to use it. This will allow you to maintain one number. However, be sure to restart the Light Phone anytime you put a SIM card in it. Note that iMessage would need to be disabled for iPhone users so that the carrier receives incoming messages as SMS (green bubble).
Second Number: If you'd like the Light Phone to have it's unique own phone number, like a "work number" phone and "friends or family number", you may get a second SIM card for your Light Phone either from Light's own plans or going directly to a carrier of your choice. NOTE: The Light Phone II in this situation would use it's own unique number, independent of your other phone device, meaning that texts/calls do not sync across devices.
Personal Hotspot: Using the Light Phone II as your primary device, you can set up a personal hotspot so that in certain situations you would be able to use another device like a laptop, tablet or old smartphone to access the internet on the go from a suitable device.
Carrier Services: Some carriers offer ways to maintain one number across multiple devices. These services usually involve a small fee added to your current phone bill. Once you’re signed up for a number share service, the carrier will send you a SIM card for your Light Phone.
Currently the phone works with T-Mobile - Digits: https://www.t-mobile.com/offers/t-mobile-digits .
It will eventually be compatible with Verizon NumberShare as well, but it is currently not supported. (Verizon is working to make our phone, and other Android devices compatible by the end of the year 2020 https://www.verizonwireless.com/support/numbershare-faqs/ )
iMessage note: Apple’s iMessage service is proprietary to them and can’t be offered on any other devices. We recommend disabling it if you are swapping SIMS. When someone sends you an iMessage at your Light Phone number or while your primary SIM card is in your Light Phone, then it will send a ‘green’ text bubble.