Country Code: +358 18 (partial)
International Call Prefix: 00
Trunk Prefix: 0
Telephone numbers in Åland use ranges owned by Finland.
Format: +358 18 XXXXX(X) and +358 457 XXX XXXX for regional GSM/3G operator Ålands Mobiltelefon (ÅMT). Longer local numbers than five digits are possible (usually with Sonera). Dominant operator on the island is the regional company Mariehamns Telefon Ab.
ÅMT has a domestic roaming agreement at no cost to the user with operator Elisa in mainland Finland. This agreement is reciprocal except for Elisa's prepaid customers under the Saunalahti brand. Finnish operator Sonera's 3G/GSM network covers both mainland Finland and Åland. The ÅMT numbers are not portable to other mobile operators.
References
Telephone numbers in Åland Wikipedia(Text) CC BY-SA