Its probable the base unit thinks six units are registered to it already. On a handset that's live, Press menu, select Setting Base, then press 7. Enter Base pin no and you'll see numbers of handsets in memory. If 123456, then it thinks all are in use. Begin to delete by selecting a number, pressing menu again, then Red button. Repeat to delete all and then start from fresh. If the handset you're on loses signal, then you've just deleted it from the base. Re-register it and continue to delete others.