Apple is to blame for many things, but the fact that if whatsapp has access to your contact list you can't start a conversation with any phone number in an easy way is entirely up to them.
This said, it's good to see that apple is slowly adding things so basic that it's baffling they are not there since years.
1. Long press on the phone number, but don't tap it otherwise it calls
2. Choose Copy phone number
3. Create a contact
4. Refresh WhatsApp by quitting and reopening
5. Find the contact on WhatsApp and call
If you're calling multiple companies for bookings or quotes, repeat this over and over.