see, Only Public ISP know to me, that support multi connection bridging is BSNL ISDN !!!
There one may bridge 2 or more ISDN lines, apart from that no other ISP support this in India so far...
and certainly its not possible to bridge between ISP A and ISP B...
How ever there are few costly hardware from Cisco is available which may do this for you, you gave to google them as because they are way my budget therefore i never took any purchase rate
anyway, this question is asked many times in this forum... search !!!
*www.thinkdigit.com/forum/search.php?searchid=275151
thank you...
*www.thinkdigit.com/forum/showthread.php?t=40488&highlight=ISDN
*www.thinkdigit.com/forum/showthread.php?t=38532&highlight=ISDN
*www.thinkdigit.com/forum/showthread.php?t=32495&highlight=ISDN
*www.thinkdigit.com/forum/showthread.php?t=35508&highlight=ISDN
*www.thinkdigit.com/forum/showthread.php?t=31809&highlight=ISDN
*www.thinkdigit.com/forum/showthread.php?t=23306&highlight=ISDN
*www.thinkdigit.com/forum/showthread.php?t=16485&highlight=ISDN
and many many more...