This is one for the future, but I can’t stop thinking about how badly mobile carriers mess up family accounts in particular, generally by making one person responsible for everything.
Having thought about it, the main thing people seem to use family or household accounts for are discounts, consolidated billing (parents pay, or for two partners it gets paid out of a joint account), and pooled resources such as data.
Support on family accounts is a nightmare though, nobody can decide who the “account holder” is and if they need the person whose SIM it actually is or the person whose name is on the billing contract. There is huge opportunity to rethink how a family plan should work.
There are also other cool things that can be done once you establish a concept of a group of people, such as a shared phone number that can ring everyone who has opted into it for use cases such as package deliveries where I would normally put a landline number for the house.
A lot of these things map on to businesses too, especially where you can offer API integrations and login with SSO/SAML. This would be ideal for business doing on-call on a separate phone, currently this is a nightmare to manage and expense while business accounts at large carriers often involve huge minimum contracts.
One of the things we did at Monzo on top of Twilio for on-call was build a floating phone number that payment schemes could call us on in the case of an operations issue, and it would use PagerDuty to update the list of who was on-call for what so the right people got the right calls. These either go to people’s personal numbers (not great), or to these separate phones that are a pain to manage.
We would have paid serious money for SIMs that could be tied to our own employee onboarding/offboarding system and a floating number that could be updated (done this way around to be resiliant to platform outages).
This is especially true if the API supported webhooks on calls, just to get something posted in an on-call Slack channel that a particular payment scheme tried to call, and then who picked up. This also helps automatically build incident report timelines.
As a bonus, if you own your own technology core, there is little to stop you from offering the ability to see the SMS log or make calls from a company laptop too, which is particularly helpful because you can use headsets for longer incident calls or phone conferences we have to join.
Even outside of incidents, the separation of work and personal phone numbers is important, particularly because when you offboard an employee or they just go on holiday, you want them to stop receiving business senseitive calls and either redirect or notify the person calling of the change, much like company email addresses. 
In general, there seems to be little middle ground for company-owned SIMs between buying Pay as you Go SIMs and going for huge contracts that also often include devices and don’t have that API or flexibility. There is a serious opportunity here for change from organisations willing to pay a small premium for easier management. 
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