In my textbook I read that in German you can say phone numbers in single digits
7-6-2-8-3-9-etc. (sieben-sechs-zwei/zwo-acht-drei-neun-etc.)
or in pairs
76-28-39-etc. (sechsundsiebzig-achtundzwanzig-neununddreißig-etc.)
I am wondering if there is any preference upon either way or they are equivalently applied.