'signal' is ok, but again it sounds a bit 'technical'. If you write something like: 'telefonen er død, det er ingen lyd i den' it's more colloquial.

"Jeg får ikke summetonen. Telefonen min er død."

That's what I would say about my home phone if were not doing a translation, but just telling somebody about it. (I don't have a cell phone.) "Summetonen" is the sound you hear when you lift the receiver and your connection is OK.