So clay does have an enrichment to find Mobile Phone Numbers. If you go to the "Enrich Data" menu and search Mobile phone you should see a few options that can search for this data for you.
We have a number of providers that we use such as Nimbler, People Data Labs, ContactOut, Datagma, there is an option for Apollo.io if you have an account with them, Lusha, Swordfish, and also we have our own Clay enrichments.
You can choose to run of the se providers or setup a waterfall enrichment were you pick and choose which providers you would like to use.