I am using multiple Arduino MKR GSM 1400-based sensor monitoring devices.
The devices work as they should with regards to packet transmissions when powered on individually.
The devices also work well when tested at the same time with different network SIM cards (e.g. lyca and giffaff).
However, when tested with same network SIM cards (both giffgaff or both lyca), one of the devices experiences GSM failures and does not transmit packets.
Also note, the transmissions fail when both devices try at the same time. If a lag is introduced between the transmissions on the two devices, they work properly with some GSM failures.
Please advice why this is happening and what can be done to sort the issue.