OK. First of all, we provide these starting digits for cellular numbers so our visitors can differentiate between a cell and a landline phone…but, it would make no sense for someone to provide you with an incomplete cell number. In other words, the number you have should include the first two digits (or 2 digit cell code as you call it). You have the correct number of digits (8 without the starting zero) which is how Cambodian mobile numbers are formatted. However, I am not aware of cell numbers in this country starting with 77. I’m not saying they don’t exist – sometimes newly introduced calling codes take awhile to be announced to the international public. So, if this is a Cambodian cell you would dial 00 855 77 477 ???.
There is another possibility I am seeing here: isn’t this actually a Qatar mobile phone traveling to Cambodia? If this is the case, than you don’t need to add any international calling codes to it.
I hope this helps. Let me know if you have any further questions.