Adamant connection error issue in 3g usb modem no web solution has worked

I come here with high expectation for a really working solution to a problem which has defeated all solutions in almost all forums which I have visited in last 1 & 1/2 months. AND I have observed innumerable posts from people facing same/similar issues and running from pillar to post without a working solution. I wish someone would take this up as a challange and help persons like me. Well, the problem (which is also well known) :

i) Hardware >>> Huawei E1752 USB 3G Modem unlocked - Latest drivers (4.20.11), firmware - CD6ATCPU (11.126.13.00.00) and software - Mobile Partner (16.001.06.01.500)

ii) Systems >>> Desk tops & Laptops Intel, win7, Xp (service Pack 3), Vista Service Pack 2 all fully auto updated daily.

iii) connection error 619 or 628 when dialed in RAS dialing mode and connection terminated when dialed under NDIS dialing mode in mobile partner. When dialed from dial-up connection set up, error 619 or 628. This happened in one case after upgrades and in one case even without any change.And earlier the modems were connecting OK.

iv) Steps taken so far : All settings, APN, Dial up number etc set correctly, AT+CGDCONT=1,"IP","APN" etc proper & correct.All solutions in most of forums tried like rasphone entry delete, APN setting, reinstall of OS, dialer software, driver and firmware update etc.

v) Modem is OK, green and blue lights flashing, AT command reply is OK, ATZ command is also OK, Modem is recognised in all - putty, mobile partner, MDMA, dial up, modem query in modem properties in device manager and query modem gives all OK except message "CD dropped--Remote modem hung up. ModemStatus=00000030". All components of data card, modem and usb composit device are properly installed. Other logs can be provided of modem, dial up and event log if required.

vi) the errors are constant and same in all PCs, Laptops and in all OS - Xp, Vista WIn7

I am determined to know whether the modem is faulty (if so, what test it should fail? because so far it has responded ok) or what is preventing connection. Obviously it is some setting in modem itself as the sim works fine if connected in a mobile phone or other modem.

In fact the time I have spent on web and otherwise for finding solution is not worth the cost of modem but I pursue for sake of knowledge and the fact that this is a very very common issue and still WHY no one has come up with a very exact solution (if all other normal remedies fail). Also in all other devices such problems are not too many why in usb modems only so many problems so that even manufacturers nor service providers are able to give solutions? People just accept loss and buy a new one.???????

Thanks in advance.