In SMS Gateway setting, you should see the following fields
Let us talk about the easier one first: "Characters to remove from phone numbers"
You may write phone numbers as +44 9876-123-456 or 44 9876 (123) (456), but you always dial it with 449876123456. As you can see, normally non numeric characters such as dash or white space have to be removed.
In the "Characters to remove from phone numbers" field, you enter the characters that have to be removed from phone numbers. For instance, "\s+-()" tells the system to remove white spaces (\s represents a white space), and +, -, (, ), [, ] from a phone number before it dials the number.
Please do note that, some SMS gateways may require the plus sign as the leading character for international call. So don't just copy and paste the above pattern string.
Now, let us talk about the harder one: "Dial Prefix"
In your user database,such as your AD, the users' phone numbers might be stored with or without the country code. For instances, Alice may have her mobile number as 07955512345, but Bob has 447944467890 (44 is the UK country code). Some SMS gateways such as Clickatell requires the so-called international phone number which requires the phone number to be prefixed with the country code. Without having to change phone numbers in your user database, you can use the "Dial Prefix" field to tell the system to automatically prefix a country code to a phone number. For instances, "1" tells the system to prefix all phone numbers with 1 (so, "7145285644" will become "17145285644"), and "44|0" tells the system to replace the leading 0 with 44 (so, "07955512345" will become "447955512345").