Or "he told me his number"...
Usually ditransitive verbs in English can go either way round, "he Xed Y to me" or "he Xd me Y", but this is an interesting case where only the second works. It's not even universal to "tell" - "he told the story to me" is fine, presumably because unlike "*tell a number", "tell a story" is grammatical on its own without an expressed recipient.
The possessive adjective a lenites nouns that start with a consonent and doesn't do anything to nouns starting with a vowel when it means "his" and it doesn't do anything to nouns starting with a consonant and prefixes h- to nouns starting with a vowel when it means "her".
A h- prefix is not lenition.
a uimhir - "his number"
a h-uimhir - "her number"