I could not find the SPA-400 user manual on the internet, and before
buying one I need to check something.

On a SPA 3100 (same as SPA 3102 but without a router) you can forward a
call from PSTN to VOIP and select a "on-hook while calling VOIP" field
in the config menus (this is only in the latests firmware releases).

Does someone now if the same can be done on the SPA-400 ?

Or if someone could point me to the user guide (and not the SPA-9000
user guide), I'll be thankfull.