Multiple call appearances are needed on phones to use features like transfer and conference. Here is a list of how many call appearances are available on a line key of a given phone. By default the phone configuration generated by a NetVanta 7100 for an IP phone will include 2 line keys. If you wish to do a 3 way conference or have the ability to answer more than two incoming calls additional line keys will be needed to accommodate more call appearances.
ADTRAN IP 700 Series
1 call appearance per line key
ADTRAN Polycom Series
2 call appearances per line key
There is no concept of a line key but 6 call appearances are available for the softphone to use
NOTE: These numbers are not an indication of the call capacity for each phone.