I say this is a bit of a vague question. It depends on what you exactly what to do with the SIP/H.323 device. The simplest use-case is that you have a SIP/H.323 device, for instance in a video-conference room, and want to use it to join a webinar instead of a regular meeting. Is that what you are looking for?
I can't say for sure that it works, but I believe that you can join a webinar in the same way as you join a meeting (see Joining Meetings from a SIP/H.323 Device ) as you just need a Webinar ID that is the same as a Meeting ID. However I'm pretty sure you won't be able to use all the features available for a Zoom Webinar, such as Polling. If I had a device I would just go ahead, create a test Webinar, start it and then try to join from the device via SIP or H.323.