If the tenant is liable for the telephone line and paying the telephone bill (i.e. it’s not included in the rent as you would commonly find with sharers) then, yes, they can take the number with them as it’s their number to take.
Special provision for telephone lines and telephone numbers is sometimes written into tenancy agreements but care needs to be taken that any such terms are fair and reasonable. Terms that are unduly restrictive on the tenant would be deemed unfair and therefore unenforceable.
