I was trying to enter an international phone number of the format
+65-12345678 into a cell in Google Sheets and it would turn into
ERROR. I changed the formatting of the cell using Format → Number → Plain text. But this error still persisted.
+ sign is treated specially when it appears as first character, there is no standard formatting that will work. We need to create a custom formatting to enter this phone number.
Click the cell where you want to enter phone number and choose Format → Number → More formats → Custom Number Format. Enter this format:
+##-########. This tells Google Sheet that we will enter 10 digits, represented by the hash characters and it should show up as shown in the format.
6512345678 into the cell and it will be displayed as