CRM Panel / Phone Number Nomenclature/Formatting

What is the proper way to save phone or mobile numbers for contacts in IFS? Please advise on :

1. Preferred Mobile Number Format
2. Preferred Landline Number Format
3. Format for saving a Landline number with an extension. (important)
4. Preferred Format for Saving International Phone Numbers

We aim to revise the phone number formats to ensure users do not encounter error messages when attempting to initiate calls through the CRM Panel due to an incorrect format.