Phone numbers can get rather long when you have to insert dialing pauses (usually represented by ","), outside vs inside access, access numbers, extensions, confirmation codes (which might include "#" and "*"), etc. The best test of a phone number is actually to dial it or text to it and verify that it is legitimate and request a return tone, text, or click through to a verification site. Yes, 7-15 seems right, if you aren't going to verify it otherwise.