If you've typed a phone number into Excel with a leading plus sign, such as +447700900000, and watched Excel either strip the plus off completely or throw up a "found an error in your formula" pop-up, this is expected behaviour rather than a fault. It happens because Excel treats anything starting with a digit, or a plus followed by digits, as a number or a formula, not as text.
By default, every cell in Excel uses General number formatting. When you type a plus sign followed by numbers, Excel interprets this as the start of a calculation rather than as international dialling code notation, so one of two things happens: it silently drops the plus and keeps only the digits, or it shows a formula error because what follows the plus doesn't resolve into a valid calculation.
If you only need to fix a handful of individual cells rather than a whole column, you can type an apostrophe directly before the plus sign, for example '+447700900000. The apostrophe tells Excel to treat everything that follows as literal text, and it won't be displayed in the cell itself.
Phone fields are commonly imported with a leading plus sign so the number is stored in full international format, which matters if you're using calling, SMS, or other integrations that expect that format. If your source file loses the plus sign in Excel before you even get to Zoho CRM, no amount of field mapping during the import itself can bring it back, since Zoho CRM only imports the value that's actually in the file. Fixing the column format in Excel first, before you import, is the only reliable way to keep the plus sign intact.