Since VoIP and SIP trunking run over your internet connection, this is a reasonable question, and the honest answer is: it depends on how your system is set up.
Answer: A well configured system includes failover options, meaning calls can automatically reroute to mobile phones or a backup connection if your primary internet drops. This isn't automatic with every provider, so it's worth asking specifically what failover looks like before you commit. If a provider can't explain their failover plan clearly, assume there isn't one.