Dears;
We have a problem connecting our SIM900 modules to a specific operator. I hope you can give me a solution or answer what you think about the problem to give me ideas of what tests I can do to establish if it is an operator's problem, of the SIM900 module, of our design, or of the simcards.
With the following considerations:
1. We have around 1500 tracking devices with built-in SIM900 modules.
2. About 6 years ago we worked our devices with the same operators in our country, there are 4:
"Movistar", (MCC + MNC 732123)
"Claro", (MCC + MNC 732101)
"Avantel", (MCC + MNC 732130)
"Tigo", (MCC + MNC 732103)
There was no problem until about 1 year ago.
3. About 1 year ago there are problems of registration with only one operator, "movistar", in a specific area where our company's facilities are, it is very likely that in other areas the same will happen but for this it is very difficult to prove it.
4. SIM900 modules are properly approved for all operators in our country.
5. The devices achieve, successful connections "+ CREG: 1", about 3% of the times they try when they are within the "problem zone" with a simcard "movistar"
6. The devices achieve, successful connections "+ CREG: 1", about 99% of the times they try when they are within the "problem zone" with a simcard "Claro", "Avantel" or "Tigo".
7. We have escalated the problem with the operator support team but have not solved anything because they say that in the "problem zone" they have the GPRS network coverage as it should be.
8. Any simcard "movistar" installed on any cell phone, for example, a Huawei P30 ELE-L29 in "GSM Only" mode, works well within the "problem zone".
9. We have a SIMCOM development module, "SIM900D_EVB_V1.01_PBC" which exhibits exactly the same behavior as our devices. Note that the development module has a SIM900D module instead of a SIM900 like our devices.
10. In engineering mode (AT + CENG = 1), the few times connections are achieved, we have managed to identify that the neighborhood actually has GSM / GPRS antennas of the "movistar" operator, According to our investigations, the first one less than 500 meters from our facilities.

LAC (Dec) CID (Dec)
5073 50382
5071 53032
5071 52083
5071 52911
5071 53033

We have tried several things, such as:
1. Force manual connection with COPS = 1.2, "732123", does not work.
2. The "movistar" operator works the GPRS network in the 850Mhz band, we have changed the bands manually to "AT + BAND =" GSM850_MODE "AND" AT + CBAND = "GSM850_PCS_MODE" and does not connect.
3. Restart is done by completely removing the module voltage and it does not work.
4. The radio of the module "AT + CFUN = 0 and" AT + CFUN = 1 "does not work.
5. We have tested a wide variety of antennas and it does not work.

Thank you very much for your support, I remain attentive to any information or log file that you require.