Activating a new mobile phone with a ported number is a snap nowadays. 

It takes about 10 minutes to activate a new mobile phone with a ported number thanks to all competitions and FCC's regulation for telephone number porting. One can use a new phone with a ported number within an hour. If the ported number was associated with a monthly plan, one may have to pay for the entire month even the number was ported to the new phone immediately after the billing month started, so it is good to port the number toward the end of the billing period.

Cricket Wireless seems to have the best pay as you go plan - $30/month with unlimited talk and 2 GB high speed data.  The 2 GB /month data are more than sufficient for most people because Wi-Fi is almost everywhere hence cellular data is not needed frequently.  Simple Mobile seems to have poor reviews due to their poor customer service. 

I used the $30/month plan with unlimited data and text (5 GB at 4G), 100 minutes talk for a long time, and I was quite happy with it until the a nor'easter knocked down a power line in March 2018, and the power outage lasted for about 20 hours.  There was no service when I needed it most.  I had to drive one or two miles  a few times to get the service to check things.  It took a little bit effort to locate the nearest spot with T-mobile service.