Why does the OPPO mobile phone set the telecom card as a secondary card and then it can't be used?

1. Telecom SIM card is improperly placed, so it needs to be taken out and correctly placed.

Second, mobile phones do not support supplementary cards as telecom cards.

Third, the telecom card is not open for use after it is placed. Take OPPO R9S mobile phone as an example. The way to open and use it is as follows:

1. First click "Settings" on the desktop of OPPO R9S mobile phone.

2. After entering, select the "Dual Card and Mobile Network" option.

3. Then find "China Telecom" SIM card to enter.

4. After entering, just turn on the switch in the "Enable" column.