Hello Jorg,
You aked: “The question that arises is, how is a sotapanna anugami then guaranteed to attain the sotapanna phala? That means he/she is GUARANTEED to get an opportunity to listen to a desana by an ariya somewhere in a future life.”
Yes. That is correct.
– Basically, the “Sotapanna phala moment” will take place even in future life.
– But one must have understood (“seen with wisdom”) the Four Noble Truths/Paticca Samuppada/Tilakkahana.