Charter Communications (CHTR -2.12%) and Comcast (CMCSA -5.82%) are adding hundreds of thousands of wireless phone subscribers, but investors shouldn't get too excited yet.

Both cable companies increased their wireless phone net additions year over year in the first quarter, something none of the big three wireless carriers accomplished. Comcast came in with a strong 355,000 net subscriber additions during Q1 but still fell short of net adds at AT&T and T-Mobile (TMUS -0.08%) for the period. Charter, meanwhile, blew everyone away with its 686,000 net subscriber additions.

So where are all these net additions coming from? T-Mobile has an idea.

Handing out phone lines

Charter and Comcast both ran promotions offering discounted wireless lines to new and existing subscribers.

Charter launched its Spectrum One offer, which bundles home internet and a wireless line for $49.99 per month, in October last year. It saw a big uptick in the fourth quarter, and that carried through to Q1. But that promotional pricing ends after 12 months, and it'll be interesting to see what happens when subscribers' bills increase.

Similarly, Comcast started offering a bundle of home internet and wireless phone for $49.99 per month for 24 months in February. That offer is not available to existing subscribers.

"We've seen cable giving away free lines that don't, by the way, appear to be incrementally pulling from existing customers and incumbent providers," T-Mobile CEO Neil Sievert said on the company's first-quarter earnings call. In other words, he's saying the customers adding lines at the cable companies aren't canceling lines at their existing providers. That means those customers might not be particularly valuable.

Charter says it's doing great

Charter touted the massive success of its Spectrum One offering during its earnings call.

"The majority of new lines continue to come from existing internet customers, though the percentage of lines coming from acquisition has increased significantly since the introduction of our Spectrum One product," CFO Jessica Fischer said on the call. "Our converged customers have meaningfully lower internet and customer relationship churn," she added.

That last part might be true as it stands today, but it remains to be seen how customers handle their higher bills in just a few months. For now, customers are receiving a free test drive of Charter's service. And perhaps that's what the company wants. And while it could possibly translate into higher net additions over the long run, Charter's going to see churn rates climb when the promotional pricing falls off.

Comcast, meanwhile, was much more modest. The cable giant plainly stated the $50 combined offer helped with net adds, and it's an example of the kind of value Comcast can offer its customers.

The impact on cable's overall business

The biggest thing for investors to watch to see if these promotions are paying off are the cable companies' total customer relationships.

If the wireless service is helping hold onto customers and bring in new ones, customer relationships should grow. However, Charter's customer relationships have increased by just 13,000 over the past year. Comcast also has 13,000 more customer relationships than this time last year. During that time, Charter's added over 2 million wireless subscribers and Comcast's gained 1.3 million. What's more, Comcast has seen its revenue per customer decline over the past year. (Charter's seen an increase in that area.)

In other words, it doesn't appear to be having the stated impact on their businesses. By comparison, T-Mobile is showing improvements in customer retention despite the growth of the cable companies. So, investors interested in telecom stocks would do well to avoid the cable companies despite their strong headline numbers.