Hi there both methods are a good sources and may have different outsomes.
For instance telephone leads can get you in the doors to pitch for sales and may have a more instant response and turnaround time. Whilst the tendering process tends is a longer turnaround, consumes time on preparation and quotes etc., can be complex if taking over existing contracts and a hit and miss strategy but worth it if successful.
Refer to Dave's post above. If you are not already cleaning or cleaning in the commercial sector you need to establish what type of service you are going to offer as there are many.