I am desperately searching the net for “TELEPHONE.INI” to no avail.
What I would like to know is the purpose of each parameter in it, and whether there is a need to tweak it in order to improve the performance.
Can somebody help please…

What's this for?

Maybe this will help.
http://support.microsoft.com/kb/217189/en-us

To the question of LLXX, the telephone.ini file (resides in %windir%) is the actual Windows file where the settings for the Dial-Up connection are stored. As far as I know, it is only used a.o. in Win98SE, it is not found under WinXP. It is a very short file containing following lines:

[Providers]
NumProviders=2
NextProviderID=3
ProviderID0=1
ProviderFilename0=UNIMDM.TSP
ProviderID1=2
ProviderFilename1=WAN.TSP

[Provider1]
NumLines=1
NumPhones=0

[HandoffPriorities]
RequestMakeCall=DIALER.EXE

A little more info here.
http://support.microsoft.com/kb/124075/en-us
http://support.microsoft.com/kb/120221/EN-US/

I see... I didn't know this since I don't use directly connected dial-up modem.

I appreciate charles’s help. Those MSKB articles certainly shed more light on the subject.
However, I would be grateful if someone could help me to get information on what each line is doing in that TELEPHONE.INI.

For example, in my Win98SE PC, I once had following line (I don’t know what program has done it):
NumLines=33

After running Tapiini.exe it becomes as follows:
NumLines=1

But according to MSKB 120221, that value NumLines=1 is meant for WinME, for Win98 it should be NumLines=2.

Therefore I am very curious to know, e.g., what that line NumLines= is for, certainly also on the other lines in the TELEPHONE.INI.

http://www.mdgx.com/newtip3.htm#TAPIINI

HTH