Minimal : smartphone on WiFi, BUT…
A large screen easier for students: rather a tablet or, even better, a laptop.
A strong internet connection (bandwidth) easier for student AND teacher: preferably with UTP/ethernet cable (= LAN connection) or with WiFi booster. Pay attention! Your internet provider (Telenet, Proximus, etc.) and the installation in your street also play a role in this. In case of disappointing results, it is usually useful to contact the provider to optimize your bandwidth.
A good playback system (in addition to the streaming device already in use) easier for student AND teacher: to be able to play background music audibly. The mini speakers of a smartphone, tablet, laptop, etc. are absolutely insufficient.
A good set-up easier for student AND teacher: to be able to clearly visualize hands, keyboard, neck of guitar, violin …
Also useful, but not necessary:
Additional webcam(s), additional micro(s), headphones (or earphones): for better transmission and reception of images and sound
Audio interface (external sound card): for plugging in digital/electrical instruments and/or micro
All known (and lesser known) programs/apps for video calls are basically OK. However, each teacher has his/her preference. Please follow the teacher in his/her choice.