PBX and outbound calls

Hi!

Does anyone have an Idea how should I configure the PBX, so that when I make a local area call from Analogue Telephone be able to do this by dialling only one Zero (0), instead of two Zeros, as now the case is?

I can make a call from SIP Client by dialing only one Zero, but using Siemens PBX have to dial twice Zero.

Thanks,

buzuku