Connectivity / SIM / Apps

Telkomsel has the best mountain coverage. An eSIM is the fastest setup; signal holds up surprisingly well around Bromo and Ijen, but expect dead zones in deep gorges and remote villages. Telkomsel (as all local SIMs) requires you to register your phones IMEI (valid for 30 days). Most Telkomsel shops can do this but ask beforehand.

Download offline maps before you go. Save Google Maps offline for Gunungkidul, Pronojiwo and the Bromo/Ijen areas — signposting is poor and data drops out.

Install Magma Indonesia (PVMBG). The official app gives real-time volcano activity and alert levels — essential before Bromo, Ijen and Semeru-area trips.