If the phone isn't getting a signal then I cant make calls anyway
Not sure what network you're with but we have three sims in our phones and a three data sim in the mifi (with an antenna). If we don't get a signal on the phone but can get one on the mifi we can then make a phone call by connecting the phones to the mifi by wifi and then making a call via wifi.
It may or may not work without a sim in the phone - not tried that.