What is the main goal of the efforts?
If it is to increase business to your local plumbing services website, in a specific geo location, you’d be better off just putting all of the content onto your main business website, tied to your Google MyBusiness page.
Using your local business phone number on more than just your main business website is going to devalue your main website authority in the eyes of Google, who has your business phone number tied to your GMB page, maps page, etc.
If you want to do lead generation for yourself and other local service businesses, relate a single site to your geo location, and then from that website, focus on different sections (Plumbing, electrical, roofing, etc) and feature the businesses you want to promote on those pages. On your true B&M business websites, just place a “Proud Member of thewebsiteyoucreated.com”, right alongside something about Angi, HomeAdvisor, Yelp, etc – other sites that have your business profile or reviews listed. That will boost the new site with authority in your market and eventually gain more visits.
Note: ALL of these sites should be on separate hosting platforms, separate IP addresses, separate WHOIS ownership, etc – if you want the highest probability of success.
G is favoring local service companies right now, even for generic trade-related queries in many cases, so it may be more beneficial to load your MAIN business site with generic, higher volume content, you want to be found with at the local level. For example, if you want to do more water heater replacements, find the generic content people look for using that specific seed phrase, publish that onto your website with local flavors in the content (cityname, local images, local plumbing supply shops, etc), and the local people will find it easier.
If it were me, and I’ve been in leadgen for a long time, I would focus on either:
1) Putting great informative content on your main local B&M business website. Or:
2) Building a single, great, geo-specific services website, and linking them all back and forth through great content and business listings.
Just my .02.