I have same problem with my Samsung Galaxy Y Pro Duos, the. The second SIM often loses the connection to the GSM network, but the phone fails to notify me about that - it. It reports network signal strength as usuallyusual, but the number is inaccessible. It happens several times a month, the last time I did not notice it for several days - very bad.
It seems this problem is common on Samsung phones, the. The Galaxy Ace I boughbought for my mother did it a few times too, but it happens less often on this phone.
I upgraded the software on both phones with no luck. I tried to hard-select the network, so hopefully I will at least get the notification.
On one SIM phones you could try this app -this app; it notifies you when GSM connection is lost - https://play.google.com/store/apps/details?id=msd.n2g.n3g.sell . I can't tell if it works though, because it only works with the first SIM and I get trouble with the second one. If it relies on info from Android, thanthen it probably won't work, because Android thinks it is connected all the time (the signal bar is on even if the SIM inaccessible).
I am actually thinking about switching the phone and avoiding Samsung and/or Android in the future, because this is obviously not an isolated problem and neither Samsung or Google seems to care to fix it.