specific sim card cannot operate with GSM boards (SIM900)

[alexxx]

I have several GSM boards, "wearing" a SIM900 modem. There is a SPECIFIC place in the room, where a SPECIFIC sim card from a SPECIFIC provider, cannot operate with NONE of the GSM boards.

I call the GSM card and a dialer is answering. Now the funny part is that not only the SIM cards from other providers are operating OK, but also another SIM card from this SPECIFIC provider is operating OK. The even most annoying thing, is that the GSM informs me that a have a full signal 4/4! Also when this "problematic" sim card is inserted inside mobile device, it is operating just fine!

I am confused. I would assume that there is a problem with this specific sim card, but this sim is operating OK when it is inside a mobile. Any ideas? Thanks.

 

[FvM]

At least for mobile originated data calls, the modem will give you plenty of information about connection failure causes.

I don't understand what "a SPECIFIC place in the room" means. Do you say, the card works O.K. in the same GSM cell a few meters aside? Did you record cell informations?

 

[alexxx]

the card works O.K. in the same GSM cell a few meters aside?

Yes! But to be accurate, it is not a few meters aside, but something like 30 meters. Same enviroment, in both places.

I got cell information from the GSM. We have three providers in Greece: cosmote, vodafone and wind. The problem is observed with vodafone card. Cosmote gives received signal level of the BCCH carrier=50. Vodafone and wind < 40. But in any case, I get the same mobile signal (4/4) for all three providers! So what's the thing here? Someone may have a strong mobile signal indication, but because of the local cellural network, the communication cannot be established? Please note that when the vodafone sim card is entered inside a mobile phone, it works OK!

https://www.etsi.org/deliver/etsi_gts/05/0508/05.01.00_60/gsmts_0508v050100p.pdf
I downloaded GSM 05.08 document from etsi.org. Something is written in page 21 about the signal level, but still it is not clear to me, how can a mobile phone shows signal 4/4, but cannot receive a call? Can it be out of network with 4/4 signal?