Phone no ok health company that gives home service to elderly

My new (within 2 months) activated Optus phone number, keep receiving (03) 5945 0800 flash phone calls. It also left a Voicemail without having a missed phone call in the list and just said 'Hi'. I didn't call anyone else except my mum and pretty sure did not give my number out.

I called back number (03) 5945 0800 spoke to a woman but did not understand her when she said the company name.