[Asterisk-Users] Integrating with existing analog PBX

[Martin Allen]

Hi.

Am new to this concept but have been requested to add VOIP capability to a 
small office phone system.

They currently have 4 standard analog lines running into a PBX feeding 16 
phones, with all the usual features,
call transfer
call hold
internal calls
etc.

would the following seem reasonable ?

asterisk server:- ( what specs )

cat5 > broadband (VOIP)
4 FXO's for incoming PSTN lines ( TDM04B ? )

4 FXS's for output to existing analog PBX ( TDM40B ? )

Leaving the existing infrastructure as is but inserting asterisk box as a 
filter between internal system & external PSTN lines so presumably a user 
could add a prefix to a number to have asterisk route the call via VOIP or no 
prefix to send over land based analog phone system ?

I doubt they would wish at this time to replace their existing phone system 
with an all ip based system ( the cost of the ip phones would seem 
prohibitive )

Assuming the above sounds reasonable, is there any way to include a fallback 
system that would not disable the existing phone system in the event of the 
asterisk box crashing/locking etc. as dead phones is not an option ??

Many thanks for any help advice, as stated in the beginning asterisk and 
tele-comms is new to me although im experienced with linux sys-admin, 
networking etc.

Many thanks
Martin