It would translate as ‘ça craint’ or ‘c’est nul’. For an activity, you could very well say “c’est chiant”.

So after thinking about it for a few days, I came up with "Dur dur le phoning" I think this conveys the message I’m looking to translate.

“C’est de la merde” works perfectly as well when you speak about an activity, a show, a movie, etc…

You mention that it sucks and that it’s a drag as in boring/annoying, or maybe a chore. I noted it doesn’t necessarily suck ass/real bad in context. It may piss the person off or not. In so many words, there are many possible choices depending on whether you focus on qualifying what cold calling is or how people react to that. A few suggestions, mostly about the latter:

La sollicitation téléphonique/commerciale/au hasard/à froid c’est (bien/fort)…
désagréable (voir aussi)
ennuyeux
emmerdant
fatigant (-)
rageant/enrageant
moche

…c’est une nuisance.

In that specific context I would use “ça me saoûle” although it doesn’t qualify the cold-calling action but the effect it has on me.

Otherwise, “c’est naze”, or “c’est relou” apply well. At least here in Paris.