Complex sentence: In 1991 Vodafone and Telecom Finland made the world's first international roaming call, which was followed Vodafone and Telecom Finland signing world's first international GSM roaming agreement.
Split into simpler sentences: In 1991 Vodafone and Telecom Finland made the world's first international roaming call. The first international roaming call was followed Vodafone and Telecom Finland signing world's first international GSM roaming agreement.