The amount of data VoIP uses depends on the mobile app. Still, it’s possible to make a close estimate. The easiest method is to check out what codec your VoIP provider uses. The most common codec used is  G.729, which consumes 32kb/s (kilobit per second). This means that an outbound VoIP call uses around 240KB per minute.
However, a call is made by two streams of information (outbound and inbound), meaning that a VoIP call uses around 580KB of data per minute (roughly 0.5MB of data). So, someone with 1GB of mobile data can spend around 34 hours on their phone making and receiving VoIP calls.
Meanwhile, video calls normally use the H.264 codec, which consumes roughly 64KB per second. So, it uses an average of 3.75MB per minute, meaning 1GB of data is able to cover around 4 hours and 45 minutes of video calls.
Of course, if your phone is connected to Wi-Fi, you don’t need to worry about mobile data.