In India ISDN service is not at all as per International standard first of all the cable reach to your home do not have capacity to carry ISDN line voltage
The Line voltage on the ISDN must be 100V DC which you will never get on the currant Jelly Filled cables in case you are having old paper core cable then you will get 100VDc currant
Event BSNL or MTNL do not follow the standard made by the engineering training centre Jabalpur
Therefor in India there is no ISDN service your are just get fooled by this serice