To use the marvelous features of my phone service, you must have a plan. Your initial plan will either be a cellular line, or a VOIP line. You would also be able to port your existing numbers to my phone service, and therefore get everything running the way you want to with all of the capabilities of my phone service. My phone service will have no store-fronts, and will not go into the extra expense of buying phones, and having a warehouse of products that hopes people will buy. Buying unlocked phones is much easier to do nowadays than it was in the past, so this is not an obstacle. I personally would go to the manufacturer to buy my phones. Also, with the home/VOIP line, any SIP compliant client will work for the VOIP line.
Cellular
Any unlocked LTE (4G), or better phone will work. Even if you choose the cheapest plan with 3G speeds, this just means that your speed is capped at a certain threshold, but you are still using 4G/5G towers. All newer cellular phones supports 5G connectivity, and likely anything that is being in use today as a cellular phone will support LTE. Cellular service will also have the added expected option of SMS/RCS capabilities. This will also include RCS for the iPhone.
VOIP
This type of line can be added to a cellular phone by downloading and installing a VOIP client, but most customers choosing this option will likely be using it for home phone services. The customer will be required to have a SIP capable device to make things work. They therefore can not use the ATA they bought from Magic Jack or Ooma. ATAs are easy enough to buy from Amazon, and beyond the initial setup, everything will be as simple as if it was a home phone. Keep in mind, that if there is a power outage, or no internet – VOIP will not work. To keep things simple, I would recommend a Wifi SIP phone.