。
I thought I'd just handle it by phone.
Literal
Telephone [at-で] finish-[volitional-おう] [quotative-と] thought [polite-past-ました].
済ます ('to finish, settle, conclude') plus volitional + と + 思う gives 'I thought I'd finish it.' 済ませる ('handle, get done with') is a practical verb for wrapping up tasks. The volitional + と思う pattern expresses tentative past intention. Common conversation pattern for explaining a decision.