German phrase
Unter ~ darf ich Sie erreichen?
Meaning
What ___ should I use to reach you?
When to use it
It asks which number to use. It is a polite way to ask for contact details.
Example dialogue
Unter welcher Telefonnummer darf ich Sie erreichen?
Which phone number should I use to reach you?
Am besten unter meiner Mobilnummer.
It would be best to use my mobile number.