Heya @m00ooo
I see two options here... 1st along the lined that @darlicious mentioned, creatively using conditional call forwarding... The "condition" for blocked or manually declined calls is *67 (ie to forward to 3063375845 you would dial *67*3063375845# and undo with ##67#). Then, any calls that you either manually decline as you see them coming in, or that you have previously entered on your phones blacked caller ID list will immediately trigger that condition, and forward the call to that number instead of your own voicemail...
The other option I'm thinking is to sign up with a cheap or free alternate VoIP service, and have the intercom system dial that number instead of your mobile number directly. Then you can just login online occasionally to update the call forwarding configuration of where you want all calls directed to -- noone else actually needs to know what the number is. One free example is freephoneline.ca, you can get a phone number in most major Canadian cities and you only have to make at least one outgoing call every few months to keep the number. You login via their web portal and can configure which alternative number to ring, be it your mobile, or your house guests.
Another cheap paid option is voip.ms. For only a few cents per minute, and less than $1 per month, you get a very feature-rich service, but call forwarding is one of them, and easy to update call forwarding via their web interface. They may have local numbers available in even more communities across Canada if freephoneline.ca doesn't. Have the intercom system dial that number for onward routing. As a paid service, you never have to worry about losing the number if you forget to make an occasional outgoing call, other than maintaining a balance as it is pre-paid.
