Zaid Alsalami, Shaykh Dr Zaid Alsalami is an Iraqi born scholar, raised in Australia. He obtained a BA from Al-Mustafa University, Qom, and an MA from the Islamic College in London. He also obtained a PhD from... Answered 2 months ago

Bismihi ta'ala

The contract must be verbal, and therefore texting will not be valid. As for a verbal phonecall, as long as all other conditions and requirements are met, it will be valid. 

And Allah knows best.