T-Mobile is selling (or has already sold) their prepaid business to Ultra Mobile, as far as I've heard. This is causing some confusion over the exact cost for a minimum number of minutes. It's up to you to shop and compare.
It is not sufficient to buy a SIM. You need to have working phone service, so you must buy whatever the minimum amount of calling they offer. For example: if the minimum is now $x per month, then buy that.
What you need: Google Voice does not port in land line numbers. To work around this the "hack" is to port your land line phone number to a mobile service. How you do that doesn't matter from Google's standpoint. But, from a procedural standpoint, you need to end up with the number ported to a mobile carrier, such that you can both make and receive mobile phone calls, and you can send and receive text messages. Porting a land line number into a mobile carrier, or doing the reverse, is more complex than a simple mobile-to-mobile port, and it will take at least 24 hours, and sometimes several days. Wait a full week after you can receive calls on the mobile phone, before subsequently porting into Google Voice.
Do NOT cancel service, under any circumstances, during these two ports, or you will lose the right use the number. The porting process will cancel service for you, on the losing carrier, automatically at the right stage of the process.