How likely is for the head of some University/Institution to ask other professors for further details related to the candidate?
Letters of recommendation often end with 


  "...If you need further details do not hesitate to contact me on...",


and this is not a useless formality. If committee members have some questions about the candidate, they are very likely to email or telephone the person who wrote the recommendation and ask them. The content of the letter matters greatly, but it is even more important that the person who signs under the letter actually knows you well.

This is the main reason why a general and vague letter does not make you much good. Sadly, some professors are too busy to write the letter themselves, and they sometimes ask their students to prepare a letter which they are happy to sign. Be vary of this line, as the busy professor is likely to forget your name the next minute you step out his door, and therefore will not be able to provide details on you if asked. This also diminishes the effect of the recommendation, if not negates it.