I was thinking along the same line as Dylan and here's working code, with some added cases for better testing:
var input = ['(315) 506-3302',
'724.256.1695',
'18566255328',
'+01 (414) 331-2475',
'2242507689',
'1-503-789-7330',
'01 714 566 5532',
'+01 806-438-4059 +01 806-577-1741 (this guy has two listed)',
'513/335-0705',
'Direct Connect 168*9674*120 ',
'14143699310',
'NONE',
'N/A',
'12345',
'Off: +1 (914) 323-5775',
'Cell: +1 (214) 649-0738'
];for (var i = 0; i < input.length; i++) {
// remove all non-numbers and keep only last 10 characters
var clean = input[i].replace(/\D/g, '').slice(-10);// check if all 10 characters are numbers
gs.info(clean + ' is ' + (/\d{10}/.test(clean) ? '' : 'in') + 'valid phone number from ' + input[i]);}
and the output from Background Script:
3155063302 is valid phone number from (315) 506-3302
7242561695 is valid phone number from 724.256.1695
8566255328 is valid phone number from 18566255328
4143312475 is valid phone number from +01 (414) 331-2475
2242507689 is valid phone number from 2242507689
5037897330 is valid phone number from 1-503-789-7330
7145665532 is valid phone number from 01 714 566 5532
8065771741 is valid phone number from +01 806-438-4059 +01 806-577-1741 (this guy has two listed)
5133350705 is valid phone number from 513/335-0705
1689674120 is valid phone number from Direct Connect 168*9674*120
4143699310 is valid phone number from 14143699310
is invalid phone number from NONE
is invalid phone number from N/A
12345 is invalid phone number from 12345
9143235775 is valid phone number from Off: +1 (914) 323-5775
2146490738 is valid phone number from Cell: +1 (214) 649-0738
When there are two numbers entered, this will only keep the last one. Here, "valid" means the last 10 characters are all numbers. You could write a complex set of regex for all cases, but this simple approach may be more robust as long as it meets your needs.
Hope this helps.
Please feel free to connect, follow, mark helpful / answer, like, endorse.
John Chun, PhD PMP 
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Hi Jon,
Before getting into the regex part I was wondering whether we could fix this issue in a different way. If the data type of your field is phone_number_e164 (which is available in ServiceNow with Dublin Release- Using Phone Number Fields - ServiceNow Wiki ) which you can choose from the dictionary field it allows you to choose a location which when specified for e.g. North America irrespective of what you type it would format automatically as +1 (XXX) XXX -XXXX.
