Instructions refer to prepaid SIM card from T-Mobile . Can another carrier's SIM card work? Why the reference to T-Mobile specifically?
Will any unlocked phone work?
The tutorial you're reading is for porting a NON-mobile number to GV; since GV accepts only mobile numbers (NOT landlines) directly, you must port the landline to a mobile phone and THEN port it from mobile to GV. It mentions T-Mobile's prepaid-SIM program because *IF* you have an unlocked (or T-Mobile-locked) GSM phone laying around, it's the cheapest & easiest short-term mobile service to (a) initiate the first port and (b) accept the phone call GV makes while initiating the second port (can't be done till the first port completes).
You do NOT have to use T-Mobile as the intermediate mobile service, but it has the fewest pitfalls. For example, I personally used my old Tracfone (which still had airtime left after I ported its number to another cellphone earlier in the month), and others have reported success porting thru Tracfone; but it's much harder to write a "one size fits all" description for "double-porting" thru Tracfone than T-Mobile, mainly because Tracfones are registered thru their underlying carriers (not Tracfone itself) and the phone itself *MUST* be a Tracfone.