When you update your phone, your carrier, or your contract, do you immediately let everyone in your contact list know that you have a new number? A couple of months ago, I updated my cell phone to a smartphone. And while the new phone performs well, there are reasons why I decided to keep my old pay-as-you-go "dumb" phone around...
When I upgraded to a new phone, I basically did it on the down low. I only gave my new number to my bosses, family, close friends, and a few others. There are plenty of reasons why I was quiet about my new number, mostly because I didn't want to receive texts in the middle of the night from spammers nor acquaintances. Let's not even talk about the calls I've gotten at 4AM while peacefully sleeping! So keeping my old pay-go phone as a second line was a calculated move:
1. Having the pay-as-you-go phone as a second line basically allows me to create a barrier between me and anyone annoying who has my old phone number. It's like one of those junk email accounts you keep around to filter out annoyances.
2. When your phone gets calls or texts in the middle of the night and there's nothing you can do to change this, then a new number will definitely give you some peace of mind. I haven't been woken up by my phone since I changed.
3. Just like a dummy email account, you can give out your second line's number to anyone you don't want contacting you on your main line. It's kind of like the mythical 555-5555.
4. Checking received calls daily or weekly is usually enough to address any matters that arise from your second line.
5. I've tested a few resting surfaces, and have discovered that if my new phone rests on a book, the vibration won't wake me up. If it rests on the wooden floor, it will. You can try and test these different places to see which is optimal to the way you use your phone.
MORE CELL PHONES ON APARTMENT THERAPY:
&bull:Improve Your Cell Phone Reception
• Smartphones As Your Primary Alarm Clock
(Images: Flickr member Steve Wilhelm licensed for use under Creative Commons and Flickr member Strange Librarian licensed for use under Creative Commons)

Commercial Flour Sa...
I've had the same mobile phone number for 9 years with no problems. I give it out freely when asked by a reputable company and it is on my business card. I have yet to have one 'spam' call or text. I'm not sure if I am lucky or if Range is unlucky.
Now emails on the other hand, that's a horse of a different color!
I get calls from an unknown number frequently and I have a friend who gets calls from the same number. Also sometimes I get texts from people I don't know. Recently I starting getting ad banners across the bottom of the screen and I never got that before.
Perhaps the author needs to look into getting Google Voice? Creates a barrier between me and unknown callers, block calls as spam, do not disturb times so no calls in the middle of the night from unknown numbers, and doesn't matter how many times I change carriers or phone numbers provided to me by carriers, my Google Voice number always remains the same. No porting necessary. I give my number out freely and never have to worry about much because there's so many ways I can control who gets to contact me.
I agree Google Voice is the way to go. It no longer matters what my 'real' number is and I never have to change it again.
Also, if it matters to you, you get cheap international calls.
I was also thinking Google Voice as I read this. I use the number for Craigslist and give it out to people freely. You can also block callers you do not want calling you anymore:http://support.google.com/voice/bin/answer.py?hl=en&answer=115089
I ported my old land number to Google Voice. Works great and only took a day of two...
Add one more vote for Google Voice. I'm on the regional board of a volunteer organization. I use GV as my official phone number and freely give it out. It's also posted on the website of our national organization. When my time as a board member is done, no worries about who has the number. I also appreciate getting a text message from voicemail messages. Gives me a good idea about who called and why without actually checking my voicemail.
Another vote for Google Voice. My initial reason for getting GV was to have a local area code for applying for jobs while I was in college. Then I started using it as a primary contact number, especially for classmates with whom I would be working on a project or two for one class only. I love that I can block certain numbers and report spam.
The pay-as-you-go phone idea is very smart though. I am most definitely going to abandon my old number (I get strange phone calls & spam texts a lot) when I switch over to a new carrier & plan, but if I ever needed a compact, night-out in the city phone (that I wouldn't mind if I lost in the chaos of traveling) or a break from a gigantic smart phone, a pay-as-you-go phone would be my first choice.