If you want to keep your established business phone number and use it with ServiceM8 Phone (i.e. let clients call the number you've always had, and have it come through on ServiceM8 Phone), then you’ll need to maintain the service for your existing number, and forward all calls to your new ServiceM8 number.
Note that ServiceM8 Phone does not support "porting" of phone numbers — if you want to keep using an existing number, you will need to maintain the service and divert (forward) calls coming to this number onto your ServiceM8 Phone number. You cannot cancel the phone service and transfer ("port") it to ServiceM8.
You should also check with your service provider if you can downgrade your plan, and if there are any additional costs associated with forwarding calls.
If your existing business number is attached to an iPhone, you can set up Call Forwarding in your iPhone settings. Go to Settings, Phone, then Call Forwarding:
It will depend on your existing phone setup and/or phone service provider as to how to divert your phone calls. It may be a matter of setting call forwarding via one of your desktop handsets, or you may need to contact your service provider.
If you use a VoIP service provider for your business phone number, you should be able to set up call forwarding through your VoIP service account login. Take note of whether your VoIP service can "pass on" the Caller ID when forwarding calls, or whether you need to enable this as a separate option.