What is the ruling on interrupting an obligatory supererogatory prayer due to a phone ringing, and how does one repent from that?

Scholars differ regarding the interruption of a supererogatory prayer after commencing it. Al-Shafi'i permitted it, while Malik and Abu Hanifa forbade it.

The more cautious approach is to adopt the opinion of those who forbid it and to make up the two rak'ahs, thereby avoiding disagreement.

However, if the two rak'ahs were vowed, then they must be made up, along with repentance for interrupting them without a valid reason.