Scraping phone number from website in C#

Hello, I need function in C# that get hidden phone number from specyfic website (www.gumtree.pl).

For example:
https://www.gumtree.pl/a-mieszkania-i-domy-sprzedam-i-kupie/bialoleka/mieszkanie-dwupokojowe-bialoleka-warszawa/1009617539640912096580509

Phone number is 504618755 when I clicked in button "Pokaż numer telefonu".

For example code in C#:

public function getPhone(string url)
{
...
return phone;
}

I need code without using Selenium.