I am currently using a local ISDN (or analogue) connection to the telco. Can I use a gateway to connect those numbers to a Hosted by 3CX instance?
No. You must port your numbers to any of our 3CX supported SIP Trunk providers for hosted by 3CX.
If you have local analogue endpoints, such as a door phone, you can use an FXS gateway via the 3CX SBC to connect those devices to your hosted instance!