Brother MFC-420CN - Manual (Page 72)

5 - 6 QUICK-DIAL NUMBERS AND DIALING OPTIONS
Tone or Pulse
If you have a Pulse dialing service, but need to send Tone signals
(for example, for telephone banking), follow the instructions below. If
you have Touch Tone service, you will not need this feature to send
tone signals.
1
Lift the handset of the external phone.
2
Press # on the MFC’s control panel. Any digits dialed after this
will send tone signals.
When you hang up, the MFC will return to the Pulse dialing
service.