Something similar happened to my mom when we ported her number from Cingular to Verizon wayyyyy back when. What happened was that the activation, even though it completed in terms of setting up the phone, didn't fully clear the back end system with Verizon because of some authorization issue. What happened was that the last four digits of her SSN had been recorded wrong when the rep created the account and then when we gave her correct last four of SSN during activation, the non-match against the record did not generate a "hard stop" but instead allowed our end of the activation to complete.
Call VZW Technical Support from a landline and ask about it.
Another thing that it could be is that sometimes, when you activate a new phone on an existing line, it takes a little while for all of the service to properly activate onto that device. How long has it been since activation?
The tech re-entered my IMEi and 4G SIM numbers and had me pull the Batt and we are full bore now. I didn't want to spend 25min on the phone and I didn't have to. It took 9 minutes from dialing to hanging up.
Thank you sir.