I have a Fairphone 2 on contract with Phone Co-op in the UK. I want to use a Three PAYG SIM for calls and SMS outside the country. I have installed the second SIM but it only connects to the network when it is installed on its own. When it is installed together with the contract SIM, it fails to connect to the Three network.
If I enable both SIMs, neither connects to a network, but if I disable the Three SIM, the first SIM works fine. If I disable the first SIM, the Three SIM still does not connect.
If I recall correctly, three uses only the 3G network, so there is no 2G fallback.
The Fairphone is Dual-SIM Dual Standby phone, meaning there is only one SIM that will have a 3G/4G connection at any time. So if your Phone Co-op SIM is the preferred SIM for 3G/4G the Three SIM may not connect. It should connect if you set the Three SIM’s preferred network mode to 3G/4G. (In short: Setting The Phone Co-op to 3G/4G sets Three to 2G, which it can’t use).
Can’t quite explain why the Phone Co-op SIM doesn’t connect in the presence of the Three SIM though - as, as far as I know it should work with any of the network modes.
EDIT: If the network mode selection is greyed out for one of the SIMs and shows 2G, you may need to set the other SIM to 2G before you change it.