If the dossier contains only transcripts of the alleged calls made to handlers etc. ( I am not certain what it contains) then it isn't credible evidence. If however the dossier contains intercepts and other related electronic evidence, it shoudl allow for voice analysis etc. to verify the identity of the handlers.
The credibility of the dossier from a prosecution standpoint depends upon what it contains.
Here is what the dossier contains.
For those who has not gone through the evidence, let me make your job easier by giving a gist of it in the order of clinching to circumstantial evidence.
1) GPS records of the sea route took by the terrorists where the co-ordinates show that they have travelled from Karachi to Mumbai.
2) Peshawar made guns obtained on all the terrorists killed and captured. One can go into the records of the gun sales to see who bought them.
3) The Yamaha motor engine found on the rubber dinghy used by terrorist to travel from the boat to the mumbai coast. the engine number indicates that it was imported by Pakistan from Japan. One can again go and look into the sales of this engine.
4) The excerpts of the phone calls between the terrorists and their masters in pakistan. The VOIP account used by these guys opened in Callphonex, an american VOIP company was done using a Western Union transaction account and the amount paid by a guy using Pakistani passport as the identification. The passport number has been provided. One can go and look into the details of this person based on the Passport information.
5) The e-mail sent in the name of 'Deccan Mujahideen' from an e-mail account which was opened using an IP address based in lahore, Pakistan. The IP address has been provided. One can again check into this who was using this IP address. There is also proof that another e-mail account opened by Zarrar Shah also used the same IP address.
6) Grenades (unexploded) recovered from the terrorists were made by a Pakistani franchise of Austrian based company. Again one can go into the details of the sales of these grenades.
7) And all the items recovered from the boat were all Pakistani-made.
And last but not least, the captured terrorist 'Kasab' confessed that he is a Pakistani and now confirmed by Govt. of Pakistan.
It is always difficult to gather this kind of evidence in any terror related incidents and one should keep this in mind. There is hard evidence as well as circumstantial evidence which I think is quite credible and Pakistani investigative agencies should pursue it to logical conclusion based on the evidence provided.