Is intention sufficient for a رجعة (resumption of marital relations) after a single divorce, or must it be uttered verbally, given that the رجعة occurred over the phone in the husband's absence and no sexual intercourse took place until two years later?

The wife was not taken back, neither by word nor by deed, until her 'iddah expired. Therefore, it is necessary to renew the marriage contract with a new Mahr (dowry), a guardian, two witnesses, and her consent. It is permissible to enjoy her immediately after the contract.