I guess with OSB you mean OpenScape Business? (Better use OSBiz instead of OSB as OSB can also mean OpenScape Branch).
Like you say you can dial out with *7xxxx
That's exactly how you dial out.
Depends on how you programmed the speed dial numbers.
Have you added the route prefix for an outside line?
Is the extension allowed to dial speed dial numbers?
Do you use a CON matrix maybe?