Is it permissible to make a break—such as answering the phone or opening a door—during ablution and then complete it?

A slight separation between the washing of the ablutionary parts does not invalidate it, by the consensus of scholars. As for a significant separation, there is a difference of opinion:

1. A view that continuous succession (muwālāt) is obligatory absolutely (the Hanbalis). 2. A view that it is obligatory, but falls away with an excuse (the Maliki madhhab and the choice of Ibn Taymiyyah). 3. A view that it is a Sunnah (Ash-Shafi‘i and Abu Hanifa).

Those who obligate continuous succession differed regarding the criterion for a significant separation: some said it is what is excessive according to custom, and others said it is when the previous limb dries in a moderate climate.

The more cautious approach, in case of a significant separation, is to restart the ablution.