Dragana, I agree ‘de’ is not fully enunciated in the final phrase - and it wouldn’t be in many cases in everyday speech. This a standard and  commonly used phrase that just ‘rolls off the tongue’ to a native speaker. I believe I can hear the ‘d’ quickly on the way into the ‘t’, but perhaps that is just because I know it has to be there! This is one of those situations where a native speaker doesn’t have to hear every sound clearly and distinctly  - it is always ‘ numéro de téléphone ‘ and the phrase is recognised without needing to emphasise each part. 

The terminal ‘e’ sound of words such as ‘je’ and ‘de’ is almost routinely dropped in everyday fast spoken French, and the ‘d’ going in to the ‘t’ is often given the briefest of attention.

Whatever our native language, I expect we have all had the experience of understanding individual words or sounds in phrases only because we recognised the words around them. I think that in B2 and C level, this is a skill that would be expected to be developing. 

I am still very much working on it, though ! 

The better you speak French, the more you anticipate what the speaker is going to say in the next fraction of a second. In this case here, the listener knows it's going to be numéros de téléphone. It wouldn't make sense without the de. And that's why it's ok to just gloss over it when you speak.