DECLARE @constraint_name NVARCHAR(MAX), @table_name NVARCHAR(MAX), @column_name NVARCHAR(MAX)

SET @table_name = 'ciacontato'
SET @column_name = 'idfonetipo'

select TOP 1 @constraint_name = fk.name 
from sys.tables tab
    inner join sys.columns col on col.object_id = tab.object_id
    inner join sys.foreign_key_columns fk_cols on fk_cols.parent_object_id = tab.object_id and fk_cols.parent_column_id = col.column_id
    inner join sys.foreign_keys fk on fk.object_id = fk_cols.constraint_object_id		
		WHERE tab.name = @table_name and col.name = @column_name;		
		
DECLARE @sqlDrop NVARCHAR(MAX) = N'ALTER TABLE ' + @table_name + ' DROP CONSTRAINT ' + @constraint_name + ';'

IF @constraint_name IS NOT NULL
    exec sp_executesql @sqlDrop