I would advise not transferring with T-Mobile at this time. I've been trying to port a Comcast "landline" (VoIP) number which my parents have been paying $10 a month for to T-Mobile Pre-paid for over a month now, and it's still not done yet. I started the process early August 2018 and it's ongoing. I will try to remember to post back when and if the port ever completes so that others know that T-Mobile port in is back up and working again. Part of the reason the port was so difficult for me was that Comcast support gave me the wrong information for the PIN, it is the Voicemail Security PIN and not the other one. Comcast was also refusing to let the number port out even when the port request from T-Mobile had all the correct information. I had to call Comcast, specifically their Port Out department and explicitly ask them to let the number port already, even though I had already called and spoke with their agents (in different departments other than the Port Out dept) multiple times gathering the information needed to make the port happen and made it clear that I was porting my number out.
Currently, there's a glitch with T-Mobile's computer system which is being sorted out. Until that's resolved, porting ability is in limbo. They're saying the port will go through once the computer system is fixed, but I'll believe it when I see it. I suggest going another route to port your number right now.
I have wasted so many hours trying to get to the bottom of this. My opinion of T-Mobile, and Comcast especially, has dropped a great deal due to this experience. I've made a mission of stripping Comcast of every unnecessary fee such as modem rental, landline, unuseful internet speed tier. I found out that my parents were paying an extra $15 on the base rate, and not even getting the advertised speed. Apparently speeds above 60, 70 MB/s Down was not possible due to the circumstances of the physical connections on the street, but they had my parents paying for 150 MB/s down for at least a year leading up until now.
Lately, it is common to wait more than 3 hours in the queue to get to a representative at T-Mobile, just to get transferred to another department (which has no direct phone number) where you have to wait in yet another queue. There are 3 separate departments involved in porting a number and each do not have the powers that the others have. So infuriating trying to get anything done with them, and every person verifies my identity all over again. So much time down the drain.
They have a Pre-paid department, a Number Transfer Center (NTC) department, and a Customer Service Specialist department. I strongly suggest you write or type down all the information you need for the port before beginning it. Have all the phone numbers, associated PIN, and collect the phone numbers you use for calling the individual departments for whichever port-out and port-in companies you go with. You may need to use all this info frequently like I did. I hope not though.