Is it possible to call Trisha?

Solution 1:

The phone works and you can place calls - you just need to pick up the speaker first.

However, the number won't work. It's just a little Easter egg for the release date of the game, formatted to look more like a real number.

01904002013 -> 19/4/2013

Solution 2:

As of September 25, 2013 the "codename:Trisha" patch/update was released to Surgeon simulator,and it added the option to call Trisha.

The scratched number on the sticky note saying Call Trisha is obviously her old number,that's why it is scratched.Read ZAD-Mans answer for more info on old number.

Trishas new number is cut in two digit pieces and scattered over the surgeries.Here you can see where exactly are those numbers hidden.

Now that you know her new number is 099326071850 you can call her. After which a new note will appear on your desk with another clue towards the secret level.