In SMS Gateway setting, you should see the following fields
Let us talk the easier one first.
You may write your mobile phone as, +44 9876-123-456, or 44 9876 (123) (456), but you always dial it with 449876123456. As you can see, some characters have to be removed.
With this pattern string "\s+-()" as the second field "Characters to remove from phone numbers", we can achieve the purpose, as \s means white space. Remind you, some SMS provider needs the plus sign as the leading character of international call, so don't just copy and paste the above pattern string.
Dial Prefix is a bit harder to explain. It tries to standardize the mobile phone format. For instance, Clickatell requires an international format "44xxxxxxxxxxx". In you Active Directory, Alice may have her mobile number as 07955512345, but Bob has 447944467890 (it already has UK country code 44). Then the prefix "44|0" will do the job, it will add 44 (the part before the pipe character) if the original one doesn't have 44, the leading zero in original will be removed, as we have zero after the pipe in the prefix definition.