When you need to answer a call that is ringing another phone you can simply dial "*25 + extension number" then press send or dial.
For instance, Janet, at extension 1000, is at lunch but her phone is ringing. You would type in "*251000" on your phone, then press 'send' or 'dial'. This would allow you to answer Janet's ringing phone.
Another way you can pick up an incoming call is if you have that person's extension as a BLF (Busy Lamp Field) on your phone, you can simply press the BLF button associated with this person's extension to pick up their ringing phone.