How likely is for the head of some University/Institution to ask other professors for further details related to the candidate? I am thinking of applying at several Biology PhD programmes and for this I need several recommendation letters. Have you experienced the situation where the head of the targeted Department asked the person that recommended you further details about you, even though the letters already contain references to your work/behaviour in the lab? How much does the content of the letters count?
Any shared experience will be greatly appreciated!
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.