Call Hold and other supp. services are probably on the TCH (but i’m not sure). It seems dangerous to allocate a SDCCH for such “long” procedures, because SDCCH resources are critical and shall not be wasted.
I think, that non-call assotiated supplementary services (e.g. Call Forwarding operations) are activated, deactivated, interrogated etc. on SDCCH. But invocation Call Hold during the call is transfered on SACCH. Call Waiting Tone is standart supervisory tone and more likely than not is generated in MS on basis of signals (SACCH) from the network.