technow wrote:client calls landline number for office x --- gets a greeting "Welcome to office x please dial required extension or dial 1 for sales , 2 - for support, etc," --- if client dials extension he/she is transferred to the extension number --- if not then the phone is forwarded to a receptionist
AFAIK, what you explain above is plainly an IVR (Interactive Voice Response) system that an Asterisk PBX system can easily do. The e-book I mentioned in my previous post
will explain how to achieve this easily.
client calls my bosses mobile number --- gets greeting "welcome to boss man from office x he is not available right now please leave a message "
Probably, this feature has nothing to do with an Asterisk PBX system, but rather a feature provided by the voicemail system from a cellphone package. However, if the cellphone has a feature to forward incoming calls to another phone or to a SIP address, then you can have an Asterisk PBX system to greet the caller through its IVR system.
and a similar system for other offices but with different numbers
For a centralized system, you can setup ATA devices on other offices to register to your Asterisk PBX system as extensions, too. Or, you can configure each office to have its own Asterisk PBX system and then peer them. The later is a decentralized system. If you want Asterisk PBX system to bridge to each local PSTN/cellphone lines, then you will need a TDM card with FXO ports on each Asterisk PBX system.