The power you can use from the telephone line is very limited when the line is unused. The power is nowhere near enough to drive the radio part making Beocom 4 wireless. The power you can use is not even high enough to drive the display on the corded telephones like Beocom 2500 etc. which is the reason for the batteries in corded phones with display.
When the phone is ringing you can use a lot of power and when the phone line is busy you can use approx. 1/6W. This is enough to drive the radio, but then your would only be able to use the telephone to accept a call... you would never be able to make a call.
Anyway, as written by others you need the power adaptor which supplies both the electronics in the base station aswell as the charging stand integrated in the base station. You could try to make contact to the seller and claim that the power adaptor is a part of the telephone and that the seller should send it to you.
Br,
Burnix