There are no restrictions preventing any local exchange carrier from offering mobile phone service, although most have not invested in the infrastructure to offer their own service.
Of coures, both Verizon, AT&T have both local exchange service and mobile offerings - but they have the financial strength to do both (in fact, the cellular busines outperforms the traditional telco).
Some other local exchange carriers that mobile service will resell someone's network, sometimes branded as their own but usually not (Qwest wireless is not a Qwest owned cell network).
Most CLEc's don't have enough money to get into wireless, and most don't even offer a partnered mobile service, but they can if they want. A couple have partnered with Sprint and/or Clear wireless but it is failry rare.
So, although there is nothing preventing it, the money involved is the one reason why some ILEC (incumbent local exchange carriers) offer moiblle service where most CLEC's don't.
