Nortel m3904 blank screen - Nortel Nertworks Meridian M3904 Charcoal Display Phone NTMN34GA70

SOURCE: how to transfer a call on Nortel Networks M3904

Use the Transfer feature to redirect a call to the appropriate person.
To use the Transfer feature to direct a call to a third party:
To return to the original call, if the transfer is incomplete:
1. Press the Transfer soft key. The other
party is on hold and a dial tone sounds.
The LCD indicator light flashes steadily.
2. Dial the DN to which the call is to be
transferred or use Personal Directory,
Redial List, or Callers List to select a
number to dial.
3. When that number rings or a person
answers, press the Transfer soft key to
connect the calling party.
If the person called is not available,
press the line (DN) key (next to the LCD
indicator) to reconnect to the original
call.

SOURCE: nortel phone m3904 won't forward to an outside line

First, you need to call your telephone company and ask them to enable this service. There may be a fee which is why it is not a standard service.