I am going to assume your are somewhat familiar with porting and the concepts covered here. Unfortunately, the varying level of details around porting can be difficult to articulate so I will leave it to you to acquire the basic knowledge before reading through my tips and caveats. As an aside, here is a summary article from the FCC on the topic of porting.
I found the particular process of transferring a phone number from landline to mobile to be very stressful and am sharing some thoughts and notes in the hopes that my experience can help others. There are a number of things I would do differently, in hindsight.
Extemely slow and grueling process
You may be able to use your phone within a few hours for changes among wireless service providers. However, porting from wireline to wireless service may still take a few days.
You're telling me, FCC! This process took 19 days in total for me to port my phone number from wireline (landline/hardline) to wireless (mobile/cellular). This process, which I thought would take several days according to various articles and service representatives at my mobile phone company, took almost three weeks.
There were a solid five days where no calls were routed to my number at all. Oof! I really hope I didn't miss anything important then.
After 14 days of seemingly no progress, I received a text message on the mobile phone saying, "Welcome! XXX-XXXX!" (except replace the X's with the actual phone number I was porting) which was extremely promising! When that text message came through I was happier than I'd felt in ages, which shows how low this process had left me.
However, that text message was followed by five more days of seemingly no progress whatsoever. That welcome was premature it seemed. In fact, things seemed worse at this point. When I tried to test call the number, all I got was silence, then three tones and, "Your call cannot be completed as dialed". I had no idea what was happening. I thought it all got screwed up and my phone number was lost to the ether.
Hindsight: I should have prepared for multiple days with no service to that number being ported. I also should have started the process far earlier than planned. This can be a very long process with flaws and hiccups. Start it as early as possible and have backup plans and realistic expectations.
Porting when signing up for mobile service is probably simpler
When signing up with the mobile provider I had the option of them giving me a brand new phone number out of their pool or else porting my existing number to their service immediately. I chose to take a new phone number from their pool, because I planned to later port my existing number to their service. The phone number from the pool would only be temporary and last for a couple weeks. I figured it would not be a major issue to temporarily use this new phone number from their pool, and a week or two later replace it with the phone number being ported.
It was my choice to defer the port of my existing phone number until I was ready. I would buy the phone one day and initiate billing with the mobile phone company, and then I would transfer my existing phone number at a later date. Simple! Or so I thought.
The result of that choice was that the responsibility for porting my number to the mobile carrier landed on my shoulders, which I see now was a mistake. When the time came and I was ready to transfer my old/existing number to the mobile company, it became a huge nightmare as detailed all throughout this article.
Hindsight: Make the port of your existing phone number a condition for starting service with the mobile company so that the responsibility is on them to see it through. Do not plan on porting your number at a later date. Ask the mobile company to port your number immediately when you sign up for their service so that the work is solely on their shoulders. This is what most typically happens when people port numbers, and I made a huge mistake deviating from the norm.
Account verification details are confusing
My mobile provider tried and failed to port my number several times. I don't understand exactly why this failed multiple times. Porting is a coordinated effort. The mobile company reaches out to the landline company to verify I am who I say I am and that I'm so authorized to port my own phone number to another company. In reality, this is a wise measure to prevent some nefarious person from stealing your phone number, but it was also very painful. The mobile provider had my information, they had my landline account number and account PIN. I would've thought that was all they needed, but apparently not.
I was told there was an "account information mismatch" by my mobile provider the first time the port seemingly failed. They told me the "account name" used for the port was incorrect, according to the landline provider. I thought the information I provided was accurate, but the landline provider wanted the "crazy long esoteric account name" that was listed exactly as-is on my landline bills, not simply my human name. Also, the account name was truncated, and the landline provider wanted the truncated name for the account information when porting. Yeesh!
Hindsight: I should have called my landline provider first and asked, "exactly what is needed to port a number from your service?" and to have them confirm the exact details of all my account information needed for the sake of the port.
Prepare to be contacted multiple times
The mobile company would call me at odd hours from odd phone numbers and say, "This is our third attempt to contact you", when in fact, it most definitely was not a third attempt. They threatened to cancel the port if they could not contact me to verify information with them.
It all sounded a bit shady/dodgy, and I felt uncomfortable and unsure that I was actually talking to a representive from the mobile phone company. They were calling from numbers not publicly listed anywhere online. As it turns out, these were all legitimate calls.
In the end, the final person who reached out to me in this process was from a third-party company contracted by the mobile company, which sounds extremely untrustworthy, but was in fact legitimate.
Hindsight: I should have assumed something would go wrong, and constantly been alert for updates on the process at all times of day from phone numbers I did not recognize.
Visit a mobile store in person
At a certain point I called my mobile company to try and determine what was happening. I was bounced around to five separate customer representatives over the course of over an hour. Although I knew exactly who I needed to talk with and what was happening, no-one at customer service seemed to grasp the process.
Words like "port" and the concept that I was transferring a number to their service seemed extremely confusing to them. At certain points the support team told me things like, "Your port is complete" (it was not. In fact, it was pending and they needed more information from me to proceed) or "That number cannot be ported" (it could be ported. It was being ported. They didn't understand the process). The support team continually transferred me to people who could not help until I was finally connected to someone who understood exactly what was happening and took down the new account information to retry the port request.
Hindsight: I should have gone to a brick-and-mortar store and talked to a customer support representative in my neighborhood in person rather than over the phone.