Hologram SIMs can have two phone numbers: one for the SMS outbox which all SIMs have and an optional inbox number that can be purchased by following these instructions. The country code for the inbox number is something you can pick when you purchase a subscription to the number, however the outbox number that all SIMs have is a +88 number. All phones should be able to receive messages from this +88 number, however in the past we have seen certain carriers block SMS messages with other area codes unless specifically requested. Verizon happens to be one of those carriers. As far as I know its the only major US carrier that does this.
WRT tools for debugging SMS, right now your best option is to email success@hologram.io where someone on our team can help you directly.