What is the ruling on ending a phone call with "Subhanaka Allahumma wa bihamdika, ashhadu an la ilaha illa anta, astaghfiruka wa atubu ilayka" by analogy with a sitting gathering?

A phone call can be likened to a gathering in terms of reciting the supplication for the expiation of a gathering at its end. The purpose of this supplication is to be absolved from the consequences of speech that might entail sin, and this applies to phone calls just as it applies to physical gatherings.