I don't have a second handset, but I'll quote from the instruction manual - Try this,
Regsiter up to 5 GAP cpmatible handsets to your OT150 base. New handsets must be fully charged.
You can use the same handset at home as well as the office by registering it to 2 bases.
1. At the base: press and hold "FIND HANDSET". You hear 2 beeps and the in use light flashes. Youa re now in registration mode.
2. At the handset: Press "OK". scroll down (with down arrow) to "REGISTRATION" and press "OK".
3. "REGISTER" is diplayed. Press "OK". Any free base numbers will flash. Enter the number of the base you want using the keypad.
4. Enter the PIN Number (default = 0000) and press "OK"
5. The handset searches for the base and when registration is complete goes into standby mode.
To test the handset is registered. Press "FIND HANDSET" on the base and the handset should begin to ring. Press "FIND HANDSET" again to stop the ringing.
As I say this is taken word for word (with a few spelling mistakes no doubt) from the instructions. I Hope that it solves your problem.
If you still have problems or you don't have the PIN number, the helpline number is 0845 226 7794, or e-mail firstname.lastname@example.org