Some phones you can do this with. I don’t think with Polycom you can. For example with Sangoma you create a DTMF button not speeddial and you put ##*+ which dials and the plus tells it to wait for more input.
Tony I tried this with an S705 (Latest Firmware) and as soon as I enter the extension it Transfers the call but the outside party hears “Your call cannot be complted. Please check the number and try again”