Sprint Nextel's 888-876-8381 phone number doesn't seem to be active. It's answered by an automated message saying the call can't be completed due to service restrictions. It provides another number to call for assistance, which connects to customer service support for a telecommunications company called Lumen. I assume they own this number and it's no longer in service. It appears the number used to be the support line for Sprint SERO plans, which were discounted plans for employee referrals.
If you need to call Sprint support, their main number is 888-211-4727. I tried to find a more direct number for employee benefits but wasn't able to. There's a web page on the T-Mobile site that simply says the employee benefits program is no longer open to new customers. If there's still a specific plan for employees and their referrals, that information is likely restricted to them. Some former employees still on the SERO plan have been directed to the general support number.
For general Sprint support, T-Mobile has kept its online help pages intact. There's no mention of Nextel anymore, though. Much of the website directs you to T-Mobile for support, and there are some basic help articles and FAQs. There used to be a Sprint Community page, but it now goes to T-Mobile's site. However, there are still some phone plans offered under the Sprint name. Anyone who needs support should go through T-Mobile, though.
T-Mobile has several options for contacting the company. Its general support number is 800-937-8997. There are also a few short codes you can dial from a T-Mobile phone. The company also offers support through its app and live chat on the website. The chat uses a virtual assistant but transfers you to a representative if you need more than it can offer. T-Mobile has more robust online support than what's left of Sprint's website. Its main categories on the support landing page are getting started, account resources, network coverage, plan support, device assistance, and business support. The T-Mobile Community hub has some blog-type posts and a forum for members.
There are still Sprint SERO users floating around the internet on various user boards. While Sprint's help page still exists, it's pretty barren and mostly directs you to T-Mobile's help page. If you need support over the phone, you'll have to call T-Mobile. The Nextel name doesn't seem to be in use anymore, either. However, I'm sure there are still some users on obscure forums.