In later version of 6.1 you can include letters in the mobile field and Vetlink will exclude them when sending. However you can still get an error if the mobile length is over 25 characters. Below query includes mobile length as well.
select CM."Ref_Number", CM."First_Name", CM."Last_Name",
C."Details" "Mobile", char_length(C."Details") "Mobile Length", C."Contact_Type", DM."Start", DP."Dry_Name" "Diary"
from "Diary_Main" DM
join "Client_Main" CM on CM."Ref_Number" = DM."Cln_Num"
join "Contacts" C on C."Number" = DM."Cln_Num"
join "Diary_Profile" DP on DP."Dry_Number" = DM."Dry_Num" and DP."Site_Ref" = DM."Site_Ref"
where DM."Date" = '16 Sep 2024' //Change Day here
and C."Person_Type" = 'C'
and C."Contact_Type" in (5)