With call waiting (network service), you can answer a call when you have another call in progress.

Activate call waiting

Select Menu > Settings and Calling > Call > Call waiting.

Answer a waiting call

Press the call key. The first call is put on hold.

Swap between an active call and a call on hold

Select Options > Swap.

Connect the call on hold to the active call

Select Options > Transfer. You disconnect yourself from the calls.

End an active call

Press the end key.

End both calls

Select Options > End all calls.