If I was having trouble hearing someone specifically because there was a lot of background noise on my phone, I would say just that:
Sorry, there's so much background noise on my phone, I can't hear a thing you're saying.
However, there are countless other things we might say:
Sorry, I can't hear you at all.
Sorry, this phone is terrible. The line's crackling really badly.
There's so much noise on the line, I can't hear you. I'll call you back later.
This phone is driving me nuts! I'm taking it back to the shop. There's so much noise on the line, I can't hear a thing you're saying.
Unless I were in a work situation where I needed to speak formally to people, I wouldn't use "There's a technical problem with the line".
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