Generally speaking the coverage for mobile phones in Crete and Greece is excellent but bear in mind that the rugged geography of the island will prevent you from getting a signal in a number of places such as deep gorges or remote mountains (when not in sight of a transmitter).
If you are going to use your mobile phone a lot for inland calls it might be a good idea to buy a Greek SIM card with a pre-paid account and use it for the length of your stay. You can get them for around Euro 20 in a number of shops. The current mobile phone providers are Vodafone, Cosmote and Wind. Bear in mind that since June 2017 roaming surcharges have been dropped within EU countries (provided of course that your phone contract is within an EU country) so getting a local SIm might not be that useful anymore.
Until a few years ago the state of the fixed telephone network in Crete (and Greece) was one of the worst in Europe. Nowadays, it is just as good as anywhere else in the developed world.