I have a German SIM card which has been great in the EU and UK. It appears advisable to get a Turkish card since we will be there for 3-4 weeks. What advice does the group have? Thanks.
With advice from the forum, I walked into a TurkCell store (near the Galata Bridge) in Istanbul and bought the 389 TL (Turkish Lira) / about $24.50 at the time, less now, Turkcell Tourist Pack which gave 20GB of data and some minutes plus free messages on WhatsApp. It worked great in Istanbul and along the entire route of our Rick Steves Best of Istanbul and Best of Turkey tours.
https://community.ricksteves.com/travel-forum/tech-tips/has-anyone-bought-a-sim-in-istanbul
There are TurkCel shops in the Istanbul airport. If you want connectivity right away, you can get SIMs right in the airport once you arrive, or find plentiful shops all over the city. I like to grab a local SIM as one of my first post-arrival tasks.