They have requested and it has been refused. The only option is to obtain a court order, highly expensive and at the end there is no guarantee they will comply.
Mark's legal team can demand these if they are deemed to be evidence of Sami being alive if someone is accused of murdering him. No cost to Mark as he would get legal aid to defend him as a miscarriage of justice. also, you mentioned that Mark paid in 'thousands' to Samis account, as a student where would Mark get this money from and do we have a copy of Sami bank statements how much he had in his account at the time of his suspected death? and do we know how much was in Mark's bank at the time leading up to his father being deemed missing?
The BT bill is the key to this case, there is no reason why it shouldn't be obtainable. It is somewhat coincidental too that Mark just happens to remove telecom equipment the moment Sami disappears.
Indeed! So, what Mark is saying is: He has no record of any phone calls he made to his father on his phone because he never communicated with his father over the phone under any circumstances? and his Father never used his phone, which he paid for, and the internet,emails? nothing.

This isn't proof that Sami was alive in October. He failed to keep medical appointments and was uncontactable after mid August.
In my opinion I would suggest Sami had many identities, passports, names, and places of cash stashed. as he comes across as a dodgy character. is Mark the only person who knew about his fathers 'strange' life?
If someone saw Samis car being driven away who did they did they say drove it and when was it returned?