[Asterisk-Users] conference call

[Altus Snyman]

Good day all
We have a Mitel 3300 connected to a grandstream handytone 486
These is a conference unit,one big speaker phone,my question is how to 
make a conference call using asterisk
Other phone has the conference button on so if you press it you can call 
someone else and all can talk together
Is there a way to do this in asterisk for example *22 <extension>
Please Help
Thanks
Altus