Block unknown callers at work, but let them through if they try again I have Tasker for Android but I don't know enough about it to be able to implement this.

I'd like this to occur when I am connected to a specific wireless network:

If someone phones who isn't in my contact list, reject the call immediately (straight to voicemail).

If they call again within a certain amount of time (say, 30 seconds) allow the call to come through.

Frankly, I can't even get the first one working. I know it's possible. The time thing is a kind of added bonus
Have you tried browsing the Tasker's wiki? There are some good pre-made profiles there you can use.

The easy way to block calls with Tasker is described here:


  Open Tasker, create a new Profile and name it "Blockr". Select the "State"
  context menu and the "Incoming Call" context from that menu. Keep this
  screen open and follow the instructions below to indicate which calls
  you would like to block on your phone.  
  
  Programming the Blocked Telephone Numbers
  
  You have the option to enter a single number, multiple telephone
  numbers, any number that is not listed in your contacts, or any number
  that is not listed as a favorite in your contacts.  Any combination of
  the above options can be included in the blocked call list.  Multiple
  numbers are separated by the / symbol which indicates "or" in
  Tasker’s pattern match.  You can also use the * symbol to indicate a
  match with any number of characters in the telephone number to create
  area code (or country code) call blocks.  Here are examples of what
  you need to enter in the Caller field of the Incoming Call context
  dialog box in order to create each type of call block list:
  
  To block a single caller with the number 123-456-7890 enter:
  
  1234567890
  
  To block two telephone numbers, enter the telephone numbers with a / symbol between them:
  
  1234567890/1234567891
  
  To block three telephone numbers, enter all three telephone numbers with the / between them:
  
  1234567890/1234567890/1234567892
  
  To block calls from individuals who are using a caller ID blocking feature enter the number zero:
  
  0
  
  To block calls from individuals who are not in your contact list, a pre-configured variable that matches all numbers in your contact list
  is used (C:ANY) with the exclamation point.  The ! indicates "not" and
  is literally interpreted as all callers that are not the number that
  you enter.  In this case, all numbers that are not part of your
  contact list:
  
  !C:ANY
  
  To block calls from both callers with caller ID blocked and those who are not listed as your contacts enter:
  
  0/!C:ANY
  
  To block a number of a specific caller and individuals with caller ID blocked enter the number, the / separator, and zero:
  
  1234567890/0
  
  To block calls from all numbers except for one caller, enter an exclamation point before the number of the caller that you want to
  allow.
  
  !1234567890
  
  To block calls from all callers except those who are in your favorite list, enter:
  
  !C:FAV
  
  To block all calls from an area code, use the * symbol to indicate that any number of characters can be matched following the first three
  digits (US area code) that you enter.  In the following example, any
  telephone number from the 123 area code would be blocked:
  
  123*
  
  Block all calls from two area codes.  Simply use the / symbol to combine the area code match in example 10.  In the following example,
  all calls from numbers with area codes 123 or 321 will be blocked:
  
  123*/321*
  
  Attach the Call Blocking Action
  
  Click Done on the "Incoming Call" context window. Select "New Task" from
  the top of the "Task" selection window that is displayed. Name the task
  "CallBlock". Click the (+) button at the bottom left hand of the screen
  to add a new action.  Select the "Phone" action menu.  Select the "Take Call"
  action from the Phone action menu. Click Done Click the (+) button at
  the bottom left hand of the screen to add a new action. Select the
  "Phone" action menu.  Select the "End Call" action from the "Phone" action
  menu.  Click Done. Click Done again on the "Task Edit" window. Make sure
  that the green check mark is displayed next to the "Blockr" profile on
  the "Profile" screen. Click Apply.  
  
  Tasker will close and "Blockr" will begin
  to actively watch your incoming calls for those numbers in your call
  block list.  When an incoming call with one of the matched numbers is
  detected, the call is disconnected and your ringer is silenced.  You
  set the actions up to briefly answer and immediately disconnect the
  call so it is not sent to your voicemail.


You can modify these instructions to add another condition which checks the WiFi network you are connected to.