So with a recent bash of "Spam" calls coming from random out-of-province and out-of-country caller ID's, claiming to be CRA, etc, I've become annoyed that if I choose to not answer or decline those calls, they are transferred to my voicemail, which then exacerbates the problem because I then get a voicemail notification and have to go delete that as well...
I have an entertaining solution using one of the "conditional" call forwarding options. 
The 'conditional' options usually default to your voicemail access/deposit number, which is 3065804001 in Saskatchewan. For a n updated list of known Public Mobile (Telus) voicemail access/deposit numbers across Canada, see this thread:
I found that the "Busy" conditional forward is enacted when you choose to "Decline" a call. This is an easily apparent option on my Android phone, but requires a non-obvious double-tap of a side button (I think) on an iPhone (I'd love it if someone could confirm exact steps for me). If you set this conditional call forward to go to some disconnected number, FAX machine, company automated attendant, or your local pizza parlour, you may find the spam calls slow down as your number becomes flagged as 'not active' or 'business' and it keeps them away from your voicemail box. Unlike 'unconditional' call forwarding, you also have a chance to screen the call, so this forward will only happen if you actively decline it. If your phone is off, or you let it ring through unanswered, it will go to voicemail like usual.
The steps:
First 'query' what the existing "busy/declined" conditional forwarding number is set to, and write it down for future reference: *#67#
Set "Call forward busy": dial *67*xxxyyyzzzz# - the letters being the number of your choice to forward to.
If you have nation-wide calling, you have all of Canada to start googling and pick a fun forwarding number from. Maybe there's a company that just happens to have part of your name announced in their autoattendent... 
"Thank-you for calling Jeffrey Automotive..."
The other options are:
*xx*xxx# Activate
#xx# deactivate & retain
*xx# reactivate to retained
##xx# deactivate and forget
*#xx# query status
xx being:
004 all variable
21 unconditional
67 when busy
61 No answer
62 unreachable
Would love to hear any good stories if anyone also implements this!
So following up, here's a few test & fun numbers I've collected you can set your call forward "Busy" to when you decline a call. You can't forward to US numbers (even with the $50 plan that includes calling to the USA) but you can forward to US toll-free numbers accessible from Canada:
800 437 7950 Thankyou for calling MCI's ANI verification System
800 223 1104 Please enter your password
416 477 0034 ANAC-repeats
418 380 0099 ANAC- French (quebec city)
855 343 2255 ANAC (Press 1 for English)
877 521-2311 Caller ID readback
800 288 2020 Welcome to AT&T. Calls recorded for Quality. ANI.
800 867 5309 The number you have dialed has been disabled
800 373 3411 Free 411
613 745 1576 NRC Telephone Talking Clock
416 395 5400 Toronto Public Library Dial-a-Story
000-000-0000 - Will play the calling parties "number not in service" / "number cannot be completed as dialed" message. This is probably the best defence against automated spam callers, as it is most likely to be logged as an invalid or unassigned number in their system...
And last but not least.....
NSFW:
877-794-7883
800-328-7448 (800-EAT-S***)
800-250-9562 
Anyone else have any more to contribute? 
@Nezgar wrote:
@Anonymous wrote:Related to the topic: For those on the $10 or $15 plans, the unconditional will consume minutes. The others like this busy mode don't seem to consume minutes.
Ah that's interesting. I would have thought that it wouldn't apply to the $15 plan, with unlimited incoming. But then I guess the unconditional call forward then converts it to an outbound call, using your minutes. I guess this is how the system works so that someone leaving you voicemail doesn't consume your minutes.
The workaround would then be to set your intended "unconditional" forward using the variable call forward unreachable, and then put your phone in airplane mode for the duration.
I think all calls that drop to voicemail for whatever reason use forwarding like an outbound call as evidenced by them appearing in the call log. It's just that the conditional ones don't consume minutes.
@Anonymous wrote:Related to the topic: For those on the $10 or $15 plans, the unconditional will consume minutes. The others like this busy mode don't seem to consume minutes.
Ah that's interesting. I would have thought that it wouldn't apply to the $15 plan, with unlimited incoming. But then I guess the unconditional call forward then converts it to an outbound call, using your minutes. I guess this is how the system works so that someone leaving you voicemail doesn't consume your minutes.
The workaround would then be to set your intended "unconditional" forward using the variable call forward unreachable, and then put your phone in airplane mode for the duration. 
