Assuming you do not have 5 handsets already, make sure that the new handset is fully charged. Carry it over to the base and press Find on the base unit. (This sends out a signal to the handset.) On the handset, press Menu and use the Up arrow to go to the line marked Settings. Press Menu to access this sub-menu. The line H/Settings should be highlighted. If it isn't highlighted, use the Up/Down arrows to navigate to this line. Press Menu to access this menu. Use the Up/Down arrows to navigate to the line Register. Press Menu again. Use the Up/Down arrows to select the base to which you want the handset to be registered. Press Menu again to confirm that base registration. Enter the system PIN for this base; the initial PIN is mentioned in the manual on page 35 and on page 54. Wait for the handset to indicate that it has been registered; it will say Registering until the process is finished. If you take too long to do the steps at the handset, the signal will stop and you will need to start over.
If you already have 5 handsets, you will need to de-register one of them. If the registration process does not properly occur and the issue wasn't taking too long, either the base or the handset is not communicating properly. If other handsets work, I'd suspect that the new handset is faulty.
BT Systems has the manual on the Freestyle 750 product site: http://btbusiness.custhelp.com/app/answers/detail/a_id/12103/~/bt-freestyle-750-user-guide
. The direct link to the manual is http://btbusiness.custhelp.com/ci/fattach/get/223683/1232386209/redirect/1/filename/BT_Freestyle_750_user_guide.pdf
. The procedure to register the handset is on pages 53-54 of the manual.
I hope this helps.