I don't know of any way around this while using HubSpot's native Salesforce connector, which is hard coded to dedupe based on contact email address. If you wanted to change this behaviour, you would probably need to switch to either a more flexible third-party, or a custom solution.
Alternatively you could:
Make all HubSpot leads pass Salesforce validation by making phone number mandatory on contact creation or form submission
This is a big problem for a lot of companies that have the two-way sync. It ends up creating duplicates and breaking the sync when the records that you merge down into aren't the master record on both platforms.
@Phil_Vallender had some good workaround suggestions that can help. Additionally, this may be something that you can fix through custom code if you have Operations Hub.
I also just wanted to share that Insycle is able to help with deduplicating across both platforms without A) creating duplicates and B) breaking the sync. Full disclosure, I work for Insycle. But here is an article that breaks down the problem and shows you how Insycle helps. Additionally, you can set up automation so that deduplication across both platforms is happening automatically on an ongoing basis.
I don't know of any way around this while using HubSpot's native Salesforce connector, which is hard coded to dedupe based on contact email address. If you wanted to change this behaviour, you would probably need to switch to either a more flexible third-party, or a custom solution.
Alternatively you could:
Make all HubSpot leads pass Salesforce validation by making phone number mandatory on contact creation or form submission