How prominent should it be?

In most cases, having the phone number in the burger menu and the footer is enough.

However, depending on your store, if you want your users to just pick up the phone and call you, then that call to action can be made sticky (a bar at the top or bottom of the screen), or prominent (with a phone icon).

In mobile design it’s imperative to link the button to the phone’s dial pad so calls can be placed directly from inside your app or webstore.

It goes without saying that you should have your phone lines properly manned. No one likes to hold forever. The data on call abandonment rates in a survey conducted by Arise is quite convincing:

44% of respondents would only hold for up to two minutes, no longer
Only 29% of respondents would hold for more than two minutes
11% of respondents would not tolerate waiting for any length of time
Older respondents are more impatient on the phone than younger ones

So, if calls are important to your store you should make the experience as painless as possible. An effective way to cut down wait times is by optimizing your Interactive Voice Response (IVR) software to handle common requests.

There’s also the thorny question of whether to outsource your call center or run it in-house. Contracting your call center operations to a third party is more cost effective for a small operation. An in-house call center gives you more control over the entire customer experience so you should have one as soon as your budget allows.

I would personally recommend dealing with inquiries in-house, which will also give you a chance to build a stronger relationship with your customers.