The Special Service Number is a sequence of digits that can be stored and used to gain access to certain features of a telephone network prior to dialing an actual phone number. It is similar to IP number and should apply from your telephone network company. SSN is useful for dialing long distance call. LDS is long distance code. For example, if you set the SSN as 17909, set LDS as 1 and set area code as 416. When you want to dial back the incoming no. 6742123, keeping press [DIAL] continuously for 3 seconds, then the phone will dial back ‘17909-1-416-6742123’ automatically. If you do not wish to enter those numbers, they can be skip to next setting.