Creating an application for mobile phone is easy for a developer …
the difficulty starts if you want to make this application available on a large number of phones:

different mobiles hardware (screen size, keypad codes, memory size …)

incompatible Java architectures (J2ME MIDP 1.0 & 2.0, DoJa …)

manufacturers specific implementations (NokiaUI, Vodafone VSCL , SPRINT…)

incomplete or incorrect implementations

distribution or local constraints (naming conventions, languages, logos…)