said by Jackyboy :
Can anyone give me the plus points on either
a. Manual switching via handset for VoIP and PSTN calls
b. Automated switching
Are you referring to certain phones, or to certain analogue telephone adapters (ATA), that can route calls to VoIP or to PSTN based on characteristics you put into the dial plan?
As "trev" said your question is vague. Perhaps you can provide further details.