Yes, if your router has two VoIP phone ports (e.g. Vigor2820Vn) you can make a call from one phone to the other locally – this is sometimes known as an ‘intercom’ facility. Lift the handset and dial 10# or 20# for the appropriate port. Do not set any dial-plan entries which conflict with these codes.
On older VoIP models dial 01 or 02 for the appropriate port