Say it BT (LSE: BT.A) has had mixed results over the decades, but that cannot be ignored. Even now, BT’s share price is not even a quarter of what it was during the internet boom over a quarter of a century ago.

Nevertheless, the latest results are hopeful. Indeed, in the last year alone this share has increased by 38%. Even after the share price augment, BT offers a dividend yield of 3.9%. This puts him well above FTSE 100 average.

Have I missed an opportunity – is it still worth buying BT shares for your portfolio?

Unequal business

It may seem surprising that a long-established telecommunications company has shown such huge price increases in just 12 months. After all, the sector is often perceived as stable.

In reality, however, it’s not just BT’s share price that has behaved erratically over the years. Business results were everywhere.

Revenue has declined in three of the last four years.

As BT operates in a mature industry and to some extent seeks to prioritize profitability over growth, this isn’t a huge surprise – but it still worries me when I look at the company as a potential investor and see its revenues essentially sinking over time.

Meanwhile, last year’s net profit of £1 billion was better than the year before, but paled in comparison to the £1.9 billion achieved just two years earlier.

An older company and it shows

There is a reason for this. BT basically has the pros and cons of business as usual.

Advantages include a huge customer pool, a broad asset base, a well-known (though not necessarily universally liked) brand, and deep expertise.

But there are also disadvantages. In some ways, BT has been tardy to capitalize on some of the more exhilarating opportunities the space offers, compared to nimbler, younger competitors.

Even in the Openreach business, which seems less closely linked to BT’s classic business of decades ago, the company has struggled. It estimates around 850,000 Openreach broadband connections were lost last year. This suggests to me that its value proposition is struggling to sustain itself in a competitive market.

The business is also burdened with pension obligations dating back several decades. These prices can rise and fall, so BT sometimes needs to set aside another tranche of cash to plug potential gaps in pension funding. I see a risk that this situation may happen again in the future.

Why don’t I buy it

In fact, these pension liabilities alone put me off buying BT shares for my portfolio. I don’t like the fact that they could add billions of pounds of liabilities to the company’s balance sheet.

I also don’t think BT’s current share price to earnings (P/E) ratio of 22 is very attractive.

As I mentioned above, BT’s earnings tend to fluctuate. Even if they return to the level of a few years ago, the forward-looking P/E ratio will become more attractive.

Based on this, if the company performs well, I see the potential for the share price to rise even further.

However, due to the risk, I will not invest.