I don't know of a simple way to allow only numbers in External Contacts...how many numbers are you looking at?
Technically, you could create Number Plans on a Site and add only the numbers (or patterns) you want them to dial, and add their phones to the site.
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George Ganahl GCCX-AI, GCP, GCSME, ICCE, ICHD, etc.
Technical Adoption Champion
Genesys
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