Is it a must that the Adhān be called at the beginning of the time of Prayer?
The matter is open and not strict. It can be called at the beginning of the entry time or at the end of it―this is as it relates to those praying in the wilderness (i.e. far from towns and villages).
And what is better is that the Adhān should be called at its earliest time and one should pray early too.
However, if they (i.e. travellers and those with a need) delay the Adhān and the Salāh and combine Dhuhr with ‘Asr, and Magrib with ‘Ishā there is no harm.
(Based on a fatwa from Al-Imām Ibn Bāz (rahimahullāh), see Al-Jāmi’ fī Fiqhil-‘Allāmah Ibn Bāz (pp. 128), also Majmū Fatāwa of Ibn Bāz)