Jesus! I will tell it one time
In modern western countries phone systems have been developed to a point where it is NOT needed to use any physical or even noticeable tap.
It's not the good old phone systems that look like noodle salad, it's absolute high-tech hardware now. Comparable to routers and maintained the same
way.
It's quite easy to mirror incoming and outgoing phone calls for a specific line but, as somebody already mentioned, tapping just one number and not a
whole line is still to be done "on-site". Depending on the line you have to tap a lot of phones to filter out what's for your desire.
As you can imagine, computerized tapping leaves no trace in the signal or even hearable for anybody.
When it comes to a single-tap, this may differ but Patriot Act and their European equivalents partly allowed to use those extended resources.