You need to put this through UL, FCC, CE and TUV testing before selling it. Depending on where you intend to sell it you might need to pay for acceptance of the above certificates. For example, Brazil has it's own certifications but they will accept CE for a fee, etc.
Our batteries come with some compliance testing. I'm not sure what other testing we have done for other components. But the batteries are the important one.
This device should have a switching regulator in order to manage the variable voltage coming out of the battery to then power the phone, which means FCC testing is required.
Generally speaking testing labs will run a series of common tests that satisfy the various regulatory agencies. In other words, you don't have to run four separate sets of tests to obtain UL, FCC, CE and TUV. I've done this dozens of times over the years.
It should be noted that if they don't themselves design any of the modules that would require certification, but instead just use already certified third party modules, that greatly reduces the amount of testing their product needs.
I've read that can get the cost of FCC certification down $1-2k and the certification time us under 5 days.
If you design your own module, then it is $7-20k, and will take 4-6 weeks. (Once you've certified that, though, then if you reuse that same module in other products it is like using a third party certified module, so $1-2k for each subsequent product).
Have you gone through compliance testing?