There are two common approaches:
One method is to keep your main number analog, set up to “forward on busy” to a local SIP DID, which handles the rest of your incoming traffic. However, the analog carrier has a limit on how many concurrent calls they will forward, in a few cases only one. If you currently have call forwarding service, temporarily set up forwarding to a test number that can receive multiple concurrent calls, to see how many can be forwarded simultaneously, before a caller gets a busy signal. Or, the carrier’s customer service may be able to tell you. Assuming that capacity is sufficient and you have unlimited local calling (otherwise you’ll be charged for the forwarded calls), this is the most robust approach. Even if your SIP provider goes completely down, you still can receive calls, though only one at a time. If your PBX goes down or your power goes out, you can have an analog phone as a backup, or change the forwarding to ring your mobile.
The other approach is to port your main number to VoIP and keep one of the other numbers as an analog backup (you might still use it for making calls, if you have ‘unlimited’ calling). You set up at the provider to fail over to the analog line if your PBX is unreachable. Although this protects you from an internet outage or a problem with the customer-facing servers at the provider, if they go completely down you can’t get any calls. Of course, if your PBX is down or your power is out, you’ll need a backup analog phone (or have the provider forward to your mobile).