Many callers who want to save their cell minutes have turned to Skype.  Unfortunately Skype is not working well with the MaestroConference system we use for Interoccupy calls.  Google Voice is a free service that works well and has been recommended by several reliable users.

“I used google voice last night to call into our call.
skype worked fine for me with Maestro from February 2010 to September 2011, but simply
doesn’t work for me anymore with Maestro.”  JB

Google Voice has offered free calls within North America (yes, Canada too) through 2011;
however, on 13 December, it extended that benefit though 2012.
https://www.pcworld.com/article/246144/gmail_calling_stays_free_through_2012.html

Google Voice service does not require a Gmail address, but one does
have to sign up for a free  Google Account.  If you have a Gmail
address, you make calls from the left vertical menu in the Gmail
Window.  If you do not have a Gmail address, you make them from
https://voice.google.com.  While there Google Voice offers an
extensive suite of services, such as inbound phone numbers and
integration with all your phone numbers, you can ignore all of them,
if you just want to make Google Voice calls from your computer browser
or smartphone mobile app.

Occupy members simply use the Google Voice keypad to dial the regular
number and pin Maestro assigns when they register for a call.  If you
have a lot of background noise at your location, use the #6 key to
mute and unmute your phone.  That is unnecessary if you are in a
relatively quiet location.

Here are some informative links about Google Voice;

http://www.google.com/googlevoice/about.html
https://support.google.com/voice
https://www.google.com/voice/rates