Field Groups lets you completely customize how fields are displayed using the "Custom Edit Rows" parameter under Parameters > Display. If you need 2 fields side by side you'd just use the following.

[field_FIELD1-NAME-HERE] [field_FIELD2-NAME-HERE]

Then set "Edit Field Format" to "None" or "Span". That should work.

With that said I'd probably just use a text field with custom validation and an input mask (to handle auto formatting) as both are supported on text fields.

The input mask would let you format it however you want and validation to validate it. I do not have an example to provide besides taking a look at that functionality in the text field and giving it a try.

The included phone number input mask doesn't support all formats though and you might need a custom one. You can use string mapping or REGEXP mapping for input masks on text fields so it's quite powerful and don't see why it couldn't accommodate your needs. Basically users would just type their phone number and it would auto format it for them.

Why does it need to be stored in the database as strictly an integer? Only way to do that would be to use an integer field for storage then use a Code field provided by CB Code Field for the display to format it using PHP.