That's not a failure of effort. You were doing the work in front of you. The gap is that the missed call has nowhere to go except your memory and a voicemail box you check when you can.
Things you can put in place today
Change the voicemail, if you haven't lately, so it sets a plain expectation: you're on a job, you'll call back, and they can leave their name, what they need, and a postcode. A lot of people still won't leave a message, but the ones who do will leave you something you can use.
When you next have a free minute, send a short text to the number that just rang. Say you're on a job, roughly when you'll call, and ask them to reply with the same basics. Save that wording on your phone so you aren't composing it in a doorway. If you can send it from the same number they rang, even better.
Keep a single call-back list, on paper in the van or in a note on your phone, rather than trusting the recents and the voicemails to be the same list. At the end of the day, go through it in order. Cross off the ones you've reached. Carry forward the ones you haven't, and don't let them live only in your head overnight.
You don't have to answer every call while you're mid-job. You do have to close the loop the same day, or as close as you can. Even a text that says you'll call in the morning is better than silence until tomorrow night.
If a lot of work also comes in through a form on your website, look at that at the same time as the missed calls, so one enquiry doesn't sit in a different place and get forgotten.
A setup built around how you already work
When you can't answer, we'd draft a friendly text within a minute: you're on a job, roughly when you'll call back, and could they reply with what they need and their postcode. Nothing is sent until you approve the wording.
Every missed call and every reply would land in one simple list, so nothing lives only in your voicemail or your head. At the end of the day you'd get one message: who to call back, in order, and what each of them needs.
We'd build this around how you already work, and keep your existing number and phone wherever we can. If a tool is involved, it would be one you own and pay for, in your name, on your card. We charge for setting it up, and for looking after it if you want us to. If we step back, you keep the number, the wording and the list.
It won't win every job, because some people will always ring round. The point is that you're no longer last to reply, and that the evening call-back list no longer has to be rebuilt from memory.