8800 or callback - what to choose to start with?

Question

Marketing

There is an online store of spare parts for laptops.
Attendance so far is mainly from search engines, 30-40 visitors per day.
Previously, there was only a live online consultant, now they decided to add a more convenient voice connection.
Actually, we decide whether 8800 or a callback widget. Calls from/to mobile. Often you need to call those who issued cash on delivery to verify their intentions.
It is clear that it is better to have both options to increase calls, but the budget is limited.
Where to begin?
callback on all-in-one services is offered around 4 rubles per minute for mobile (1.95x2)