"When you arrive, text me!"
Translation:Kiam vi alvenos, tekstmesaĝu al mi!
Since we're using "when" we're clearly talking about either a past or a future action.
The past of "to arrive" is "arrived" though, thus it must be a future action.
It's a one time event in the future.
"Kiam vi alvenas" would refer to a repeating event:
"Kiam vi mangxas cerealajxon uzu kuleron" - when you eat cereal, use a spoon.
Feel free to say it, then :)
It should at least be accepted as an alternative in this sentence, I think, if it's not.