This one is worth saying plainly: it is often not an SEO problem, and an agency that takes it on as one is taking your money for something it cannot fix.
What search can be responsible for is attracting the wrong caller. What it cannot be responsible for is what happens in the ninety seconds after the phone is answered.
Work through it in this order
Listen to twenty recorded calls.
If you have call tracking you already have the recordings. This is the single highest-value hour available and almost nobody spends it.
Owned by Tracking and attribution
Are the callers in your service area?
Out-of-area calls point at pages targeting places you do not serve, which is a content and local problem together.
Owned by Content
Are they asking for work you do not do?
Pages written for services you no longer offer, or never did, produce exactly this.
Owned by Content
Are they price-shopping a job you are not competitive on?
That is a positioning decision, not a search one, and no agency should charge you to fix it with pages.
Owned by Content
How many rang out unanswered?
Call tracking reports this and it is regularly the whole answer.
Owned by Tracking and attribution
If the calls are in-area, for work you do, at prices you compete on, and they still do not book, the search work is doing its job and the problem is downstream of it.