Hello everyone
The book “The Future of Telephony” said : If you do not need to connect to the PSTN, Asteriskrequires no hardware other than
a server with a Network Interface Card.
Through only a Networkcard can I call any other kind of telephone in the world? or it’s up to the service provider?
yeah, its up to the service provider who you can call. You can also link into free systems like Free World Dialup and call those people.
If you sign up with a VoIP provider, make sure they offer BYOD and do not make you use their ATA, or it will be mostly useless for you.
Hi,thanks,you reply so quickly 
I still have another question, if I have already connected with PSTN and now want to enlarge the capacity from 10 lines to 20 or 30 lines, the new phones are IP phones, but the service provider dose not support to call freely other kind of telephone, so will the new pbx worse than before? because the new IP phones must still go through the pstn interface to outside ,and that will cause traffic jam. am I right?
your question is somewhat unclear. It really depends on the type of PSTN connection you have now and what kind of phones you have, the more detail the better.
Making a guess to try to answer your Q-
Assuming you have 10 POTS (analog) phone lines, and are using normal analog phones that plug straight into the line, I’m guessing your question is will replacing the phones with IP phones and putting asterisk between them and the PSTN create any problems?
The answer is generally no. The only traffic jam type problem you’d run into here is if you have more people wanting to dial out than you have lines, in which case when all the lines are busy the next guy that tries to dial out will get an error. However this isn’t any different from what you have now, if you have 10 phones and 10 lines, then the 11th guy who has to use the phone will have to sit and wait. And while you get the same outbound capacity, it’s actually more useful because while people 1-10 are talking on outside lines, people 11-12 can talk to each other (internal IP call will not need an outside line)
There are of course other issues to consider, for example your office LAN has to support the bandwidth for the IP phones (it wont be a problem unless your network is overloaded), if you are using VoIP providers, your Internet connection has to support the bandwidth they need (this can be improved by using a low-bitrate codec like GSM, iLBC or G.729 to compress the audio), etc etc etc.
I want to continue the topic 
So, If I installed a * server, which stay between PSTN and phones, then what is the advantage of using *box in view of the cost,since *box can not call the phones of PSTN without the supporting of internet service provider(is the ISP same with VoIP provider?)