"I do not have your number."
Translation:Níl d'uimhir agam.
All present tense verbs are negated with ní - ní ithim, ní léann sí an nuachtán, ní thiomáineann sé an bus.
In the case of tá, the dependent form (used after particles like an, ní and go) is fuil. an eclipses, so you get an bhfuil when asking a question, but ní lenites, so you get ní fhuil, and, because fh is silent, ní fhuil is pronounced níl, and that ends up being how ní fhuil is now written - just níl.
ní is also the negative form of the copula is, but it completely replaces the copula whereas the negative particle ní comes before the verb.
To re-iterate, ní is used to negate all present tense verbs, but there is a special case with tá, because the negative form has collapsed from ní fhuil to níl.
Note that ní fhuil is never used in real life, it is always níl, I'm just using it here for illustrative purposes. In Ulster Irish, where some people use cha as the negative particle rather than ní, you will still find chan fhuil used.