It depends on the phone you're using. Unlike analogue telephony, VoIP phones don't use tones or flash hooks to signal switch lines, instead the phone manufacturer will develop their own method to switch between phone lines and then use the standard SIP protocol to relay this information to Axon / the PBX.
For some IP phones and soft phones there are 'line appearance' buttons which allow you to switch between virtual 'lines' as needed (e.g. when a call waiting call comes in, the 2nd line appearance button flashes, and user can switch to the call waiting call by pressing the flashing button). On other devices, the old 'flash hook' (hanging-up and quickly picking up again) can trigger the device to relay a line switch to Axon, or sometimes there will be a clearly labelled 'Flash' or 'Link' button on the phone itself you can use to switch calls.
Note 1: Not all IP phones or devices support multiple lines/call waiting (in VoIP telephony, Call Waiting is the same as having a 2nd line appearance for incoming calls)
Note 2: Some IP devices need to have their Call Waiting feature enabled before it can be used